The most important aspects of pay-per-click are picking the right keywords and having compelling ad copy.
Keywords
Research your keywords thoroughly - you may be surprised at what searchers are actully searching on. Frequently I have found when working for clients on their
pay-per-click campaigns it is not the keyword the client has assured me would be the one everyone will be looking up!
A good tip is to pick keywords with a relatively high monthly search but a low competitor rating as you cost per click will be lower.
Ad copy
Make sure your ad copy speaks to the audience you are targetting and make it clear what it is you are offering to avoid costly clicks from searchers who are not looking for your product. For example if you are marketing a software download that costs money, make sure this is made clear in your ad copy to avoid the freebie hunters from clicking through.