Hi PMC,
Lag is a big problem for me and pretty much everyone in MC at some point. It sucks. Sometimes, lag is on your part. You can fix that with 4 easy steps. When it isn't, you have less control.

How to fix lag that your computer created:

Step one: Delete all the crap you've downloaded on your computer. If it's even kinda old, it won't run correctly. If you get a new computer, download 1-2 texture packs only, and only a couple mods, and don't really download anything else minecraft-wise.

Step 2: Get optifine. Wow, it helps. With optifine it creates more options to reduce lag. REDUCE EVERYTHING THAT CREATES LAG! No, it isn't nessisary to have everything looking amazing unless you have a really good computer.

Step 3: Your lag should be better now, but you can set a higher priority. When you have MC open, just do ctrl-alt-delete and open task manager. Select services. Now click on "java.exe" or something like that. You can set it to high. That should reduce some of your lag.

Step 4: Don't go all out in stuff you add to MC. Mods and texture packs are very lag creating. If you have 10 Texture pakcs and 9 mods, you'll be lagged out. Also, don't get a 256x256 texture pack if you're having lag. It'll probably give you about 1-5 FPS even with the steps above. Try to get a smaller based texture pack.

Now sometimes it is out of your control. If you get on a server and it's lagging, and everyone else is lagging, it's on the servers side. To reduce lag there, tell them that they have lag. If the spawn area you are in is MASSIVE, tell them maybe to make it a little smaller. And if they have hundreds of unnessisary plugins, tell them to delete a couple. It'll change the lag, trust me!

I hope this helped your lag!

Char