Post-6, Ali has really great dives. When you see it coming, just run! It's better to cost yourself some gold and experience than let Ali get to you under tower.

If Alistar is camping your lane, you may lose lane, but your team will have an bigger overall advantage (edited for clarity). Your jungle can steal the enemy jungle and gank other lanes, and the Alistar's team will miss out on gold and experience because they have to share the resource stream. Just hug your tower, run when he comes at you, and don't die. If you die, they get the advantage back. If you just stay as safe as possible, farm minions near your tower, and stay alive, your team will win overall.

No need to call people noobs when we're all here to learn and help each other.

Easy way? Ban him--he's freaking OP right now and probably the second best jungler ATM.

Hard way? Ward early if you're bot...if top, just expect it.

As was said earlier, if I'm expecting early ganks, I take an escape at level 1 if my champion has one. I'm not much of a jungler myself and I don't always know jungle patterns, so I used to always take an escape at 1, but I found that I sometimes get a little behind if I do that every time. So, basically, if you know to expect a gank, use that knowledge to your advantage, and if you don't know when to expect them, you might try erring on the side of caution at early levels.

I notice many supports on bottom ward the river brush. I find escaping a lot easier if I ward right on the corner of the closer dragon wall. While I can't see inside the bush, this gives me a little more vision of the river, a little bit of the entrance to the tribush, and the tip of the entrance to the blue side of the jungle. We have a bit more time to escape. If we are overextended and the jungler tries to close in behind us, it usually still ends in a negative health tradeoff, but that's better than a death.

I assume you were purple side on top lane since you said he came from behind, there are a few options you can do. Also, I'm assuming you KNOW he's going to gank top and not mid (maybe because of a favorable match up or whatnot).

1) Buy a ward instead of 2 health pots (if you go boots + 3 pot)
Ward vs 2 health pots is a difference of map control vs. lane sustain

2) Help your jungler do wraiths/red and have him set up for a countergank
If your jungler has decent fighting power at lvl 2, he can be on par vs Alistar since he has red buff and not blue buff. The downside is this hurts his jungling if he cannot sustain. Also, some champs like Amumu would be severely hindered to start off red.

3) (As RiotTiza said) Sacrifice your first wave of minions -- stay in range for exp, but don't risk getting the last hit
Essentially, by sacrificing your first wave of minions you are much closer to your tower and have near full HP. Another way to think of this is one wave is about 110 gold or so. By giving up this wave you are saying "In exchange for this wave of creeps, I bought a ward and health pot."

Also, if he does go from blue to top on blue side, try to let your jungler know so he can counter jungle as well. Hope this helps.

As someone who plays Ali as my main jungler, I can say that the recent nerf to his passive was huge to his early game. He takes much longer to clear the golem so he needs a stronger leash or more time. He is much slower at clearing wraiths/wolves. It's not even worth it for him to try killing small golems.

Early Alistar ganks are best dealt with by your own jungler. Ideally you have a fast jungler who can just intercept him and kill him 1v1 while he's trying to get into position. The alternative is to just gank before him, even if your jungler isn't such a strong ganker. Let's say you have Lee or Shyv. Ali will start blue, you start red. You will finish red before he finishes blue. Immediately go for a gank into mid or the closest end lane. You probably won't get the kill, but you'll either pull Alistar to help the lane or you'll provide protection from the Ali gank. As a result, only one of your lanes has to be careful (the one close to their blue that you didn't gank.)

Ali is great at diving at 6, but if your jungle is playing well, Ali should be hitting 6 at around the time your jungler is hitting 8. Take advantage of that. Ward wraiths and steal them every time. Camp mid so their mid can't take the wraiths or help Ali take them. Steal his red if you can. The best way to do this is to send mid lane or support to help your jungler. If you successfully deny him red (obviously don't let him take your blue, and your red is already taken care of) he will be even more behind. He can't do small golems at any normal speed, so he will have to rely on wolves only to get him from 3 to 6. That will take a very long time

One final thought: In team fights against Alistar it helps a lot if your team has a disengage. I use Janna as a counter to Ali, and it works great. Fight until he ultis, disengage. Now Ali is guaranteed to die if he tries to fight, and you're much better at disabling the team than he is.

Riot has tried to balance gank strength with clear speed. On most junglers, it's at a reasonable place, but Alistar's gank strength is so great that he'd have to lose all clear speed (no passive damage at all, reduced base armor) to be balanced with his gank strength. I expect that there will be nerfs to his ganks coming in the future. The best place to hit is his W. Make it a skillshot which hits first enemy (not always champion) and moves Alistar even if he misses. Alternately, his Q loses the knockup and just becomes a 1 sec aoe stun, like Riven's W.