There are a ton of tools out there to choose from, so you might want to take an hour of your time to write a list of what you'd like to be able to capture about your clients (the obvious contact info, last date of service, whether they contract for annual service, prices, codes for what was done...). It might not hurt for you to start with an Excel spreadsheet to think about each piece of data you want to have. It could help you evaluate the solutions out there.
Also, if you have a website for people to contact you, you might want to consider a software that integrates with the contact or sign-up forms so you automatically get some prospect information you can follow up on. It could give you a leg up on new business when times get too slow.
Ideally, you want to look at a solution that is not too customized (I advise people to avoid doing purpose-built Access databases and the like because if they contract it out to a specialist who then disappears from the scene, you're stuck with something no one is comfortable updating or fixing).
Good luck!