I'm betting you've already heard this, but:
---A good, hard boil followed by a fast cold break when chilling.
---Some Irish moss during the last 15 minutes of boil.
---Maybe some gelatin in the secondary (I've not tried this yet)
---and most important, but the hardest (if not the esiest)... TIME, let it sit for a long time in the cold.
There are many other factors too. For instance, high flocculating yeast, and mash procedures. If you want more info I can look into an article that I can find.