Couple IT consulting models that come to mind
- Hourly - Typically an initial quote or estimate is still needed
- Fixed - If it's something repeatable you do often. Setup a new server, re-install a desktop, build a fixed spec website then fixed can be very lucrative if done right. Don't do it on risky projects, only things that you can do with consistent outcome.
- Fixed weekly / bi-weekly if you are doing something like agile software development
- Managed Services - or a monthly fee they pay you to make sure there systems are working and running well. If everything is working well you make a lot, if it's breaking and has lots of issues you lose.
- Retainer - Is a tricky word because it usually implies that you will drop everything and attend to a client ASAP (like a retainer with a lawyer). You would want to setup a fixed amount of time you can work on their account without getting approval from anyone.
*Pre-paid - Some companies pay in advance for blocks of time. Can be used over a month, year and it gets re-filled as needed.
Managed Services is pretty popular right now - IT providers like to know that they are getting a fixed amount of income each month in and they can count on that revenue. Clients like it because they know for a fixed amount of money they will be taken care of.
'Break / Fix' is a term used when clients just call when it's broken, you charge them hourly to fix it. It's a common model, but unpredictable and you aren't really rewarded for doing an excellent job. If you do a great job your reward is that your client never calls or gives you another penny again :] So - think of something of value you can do every month for clients for some known recurring revenue.