You don't have to have 21 points in the Utility tree
9/0/21. 0/9/21. 8/0/22. 1/17/12. 9/9/12. 1/3/26. 1/14/15. 1/4/25. 1/11/18. 1/13/16. 1/15/14. 1/22/7. There are a lot of legitimate mastery setups you can take when you support, and there are probably other ones I've missed.
Looking at the advice and pages of some high level supports shows us a small silver lining for what it means to support. Whereas the pool of supports is the smallest of the five roles (this despite introductions at useful novelty picks like Yorick or Fidd by RoG's very own Elementz), the number of viable mastery setups that is much larger compared to the other four. AP and AD carries pretty much always have 21 in offense, with the other 9 being the other wiggle room.
Rather than try to tell you what you should always be taking on which champ or which builds I prefer or why, I'm going to analyze the different mastery choices from a support point of view, addressing their pros as well as cons where applicable.
Offense Masteries for Supporting
From left to right, we start with the least used tree, offense.
Summoner's Wrath, or
Summoner's Wrath is the most important ability here as you will almost always be taking exhaust. The armor reduction can mean the difference in a team fight for much of the game. The 1 point in all of those 1/x/x mastery combinations? This boy right here. If you aren't taking exhaust (and you should be), don't take this--and you probably shouldn't be in the offense tree in the first place.
Arcane Knowledge, or
If you're going to stay in this tree it's almost always because you want Arcane Knowledge. Take this if you are champ that is poking and intending to do a lot of damage. The cooldown masteries required to get it are Sorcery , which gives you cooldowns. Supports generally have medium/long cooldowns on their abilities. 8-0-22 is viable for healing/shielding champs that you aren't trying to do damage with, like Soraka, Janna or Taric.
You don't want the minion damaging mastery as you aren't cs'ing. The effectiveness of damage runes is much greater for poking and harrassing than ap are, so if you're going to further up the tree, take the damage points. Lastly, if you are doing something weird at low elo where you are a harrass build and you know you can get away with it, you can also put 9 points into weapon expertise if you know you're going to be doing a lot of ad poking and harrassing. This is not something I ever see at competitively play, but it can work at regular and lower/average levels of play.
Defense Masteries for Supporting
This tree can be your biggest tree when supporting, especially if you're running tanky or similar, like Leona.
Lines One and Two in Defense
If you dip into this tree it should be to get armor to endure the auto-attacks and the physical abilities the other lane is going to throw at you. You don't want to be the support that is dying all the time because you can't run and are too squishy. 3 points gets you 6 more armor, and another 3 can get you MR. At the second level, you can get hp/5, which will help you to do more trades, and some health.
Veteran's Scars, or
If you get 9 points in defense you should be grabbing this. This alone gives you 30 health, which is the equivalent of more than one flat health quint and more than 5 flat health seals, health's primary rune. The required durability points give you 6 health per level, which means at level 6 you'd have 66 more health. How many times does a support barely survive with only 66 health or less? Every other game, maybe? And, with this ability, you can still have your 21 points in Utility.
Past Veteran's Scars, or , , , , and beyond
Once you get past Veteran's Scars you are giving up cooldown points and an awesome 21st point, Mastermind. So, you have to be playing a game where the cooldown of flash, exhaust, and your abilities can be given up and aren't as important because you are using these instead. Indomitable continues the tankiness trend, and is very helpful if you are going to be an all-in or tanky support, or one that trades frequently even at higher levels. Alistar, Taric, Leona, Yorick, Galio, Shen and Sona are all candidates for this. Initiator is more efficient than utility's swiftness for giving your roaming and indeed initiating movespeed, so if you plan on doing any of that points are great there, and I generally recommend you get them if you're getting indomitable. Siege Commander is interesting and some people may like it, but for me towers are taken down by carries or won teamfights and it won't make a difference. I don't recommend Enlightenment or anything past it because it requires you to give up the most important mastery for supports, Greed. Furthermore, Enlightenment is scaled cooldown and flat is better for supports. The cooldown options in the other trees are much better.
Utility Masteries for Supporting
Here's the most obvious tree, the great biggy.
Lines One and Two in Utility
Pretty simple: you will pretty much always be taking flash and you will always need mana and mana regen, so 7 points are pretty obvious. Swiftness is nice depending on what your pick is and how quickly you are planning on getting boots. Scout, which gives you larger ward vision, is underrated, and I recommend learning about what it can do. It's only one point! If you're going to be a ward-happy player or you think the game is going long considering getting it.
Lines Three and Four in Utility, or , , , , , and
Greed is the most important ability for a Support. This is the reason you should about never have fewer than 12 points in the utility tree. The spell vamp doesn't match what supports do, and you aren't planning on taking any buffs. Wealth is if you anticipate a very long first lane before your first recall--it's not a cost-efficient mastery but it can fit some styles of play. Awareness helps a lot as supports are always behind on levels. Sage doesn't appear to give much, but consider it if you have an aggressive lane. One point of Awareness requires you to get 3200 experience to get 40 bonus xp. Sage, on the other hand, gives you that 40 xp for every assist and kill. So, if you get even 6 kills/assists, you will have about equaled the max potential of one point of Awareness, because it takes 19,720 XP to get to 18. If you're going for bonus xp gain, I'd generally recommend getting the one point in Sage and 3 in Awareness if you aren't devoting 5 points to it.
It should be noted that the majority of your decisions are located in these lines and that this is where you should start your build. You should always, always be taking all four points of Greed, and then go from there. Do you want to get to 21 points in Utility? Do you want 9 in defense? 1 or 9 in Offense? 9 in both offense and defense? Whatever you do...always take Greed.
Lines Five and Six in Utility
If you stay in the tree, you obviously want the cooldown over the regen and the final mastery, which reduces your flash cooldown by over 30 seconds. Strength of Spirit is okay in the laning phase--strictly speaking it's better than the regen you see in the defense tree. However, to take it you either sacrifice high cooldowns, the magic pen from Arcane Knowledge, or the health from Veteran's Scars. In the mid and long game if you are focused on teamfights, Veteran's Scars is better. Also, all supports will have Philosopher's Stone, and many have healing spells. Think of it this way--a Blitzcrank with 2,000 mana will get 20 hp/5 from this, the equivalent of 2/3 a Shurelya's and 4/3 of a Philosopher's Stone. And this is after you manage to get to 2,000 mana, which will be later in the game.
The Old Man's Trees
I gave large listing of different mastery numbers and would feel lame if I didn't illustrate a couple of them. Here are a couple of the ones I use, 1-17-12 and 1-4-25.
Good luck with supporting and have fun with the different builds you can do.
This week's eyebrows:
Old Man Eyebrows