My terms are net 30. I start charging interest after that time period. Try doing a forum search for payment terms. I think you will find several interesting threads on this very subject.
Do you let them know that they are overdue? Did you tell them in advance that you charge interest after a certain point? I would make sure you do that in the future and that should help to get paid faster.
I have payment terms spelled out in detail in my service agreement. My current terms are Net 15 and then I do charge late fees (luckily, haven't had to enforce the late fees as of yet).
I have a Net15 then charge interest for the next 15. If payment isn't received in 30 days, the contract is null and work will cease with that client until all payments are paid. That's just me though. I've been taken before and my time is valuable.
I'm lucky that I have never had to impose late fees yet! I too have it written into all of my contracts that pament terms are Net 15. I've had one client that I had to remind at the 14 day mark, but it has never gone further than that.
Nearly all my clients are prepaid package clients so I never have to worry about that. All my clients have paid me in a timely manner. Since I used to work in collections, I know how to make sure to get paid promptly. It helps if you require a deposit up front for any project work.
It is important to have all your biling terms spelled out for clients. From experience most clients are on a 30-day cycle. I have worked with a few that on on a 15 but mostly it is 30.