On meta? Pick a theme... It could be flour-based breakfast snacks, or miniature equines, etc. Wax lyrical ;-p
On SO/SF/SU - simply answer questions well. It is possible to make lots of rep by asking a spectacular non-wiki question, but it is harder.
Rinse, repeat. Rinse, repeat.
You could also peruse the bounty board.
If you are going to hit the rep limit, then knowing when UTC midnight is (in your local time) helps, too; getting 50 upvotes counts for nothing if you are maxed.
Don't post replies that look like spam or which are clearly offensive. In addition to downvotes, you can get hit with a 100pt fine. Likewise, snappish / unhelpful replies ("lmgtfy" etc) can attract negative attention.
Don't be abusive to other site users (or any other kind of site abuse); being put in the penalty box will cripple your ability to gain rep for a while.