2.Look for new players who might need an alliance
3.Advertise your alliance in chat but not constantly.
4.Don't Be a pest when you ask.

Some other things to do to help you some what.
1.If you have a massive alliance like 50 then of course you may be stronger then some one that has 25 people but it would be harder to concordant an attack on something.
2.make sure you make some rules to your alliance but not a bunch no one likes a control freak.
3.if you want to boost some peoples performance you could make a contest for a hire position.
Sincerity Jon