Small business owners: How do you get paid quickly?

I am in the process of starting my own business. I have done a soft launch and have a few clients. These clients have been regular and keep giving me work to do which I appreciate. The problem that I have is that they take 20 to 30 days to pay me.

Right now I don’t need the money because I have a full time job so I have a steady income source. But ideally I would like to be able to do my business full time. That requires me to get paid so that I can in turn pay my bills.

My business is IT consulting and support services for small business.

Is it unreasonable for me to expect payment faster then 30 days?

If you were to drop your computer off at Best Buy to get it repaired or updated they wouldn’t let you leave the store without paying for it. For my clients I don’t mind I want them to take it home and check to make sure everything seems legit before they pay me… but that should take 2 or 3 days at the most… not 30.

So: How to get faster payments?