two ways to go about it. you can either put a per project rate or a daily rate.
a per project rate works when your clients aren't too fussy about their flyers and brochures. here you can estimate how long it will take you to finish a project and get paid for it.
the daily rate works well with clients who like to revise a lot. this way, no matter how long they want to drag their projects because of revisions is up to them. you on the other hand will be fairly compensated.
Ok, now for both, you have to be quite adept at estimating how long it will take you to finish a certain project. for example a borchure will take longer than a flyer. some flyers can take as short as an hour to do. so you'll need a base rate.
aside from that, you'll have to factor in their revisions and comments. and if your end product is printer ready, you should be compensated for that.
a decent formula I use is to think of a generous (not ludicrous) monthly salary. now with this monthly rate, calculate how much your daily and your hourly rate is. so for each project, you can quickly estimate how much it will cost your clients for how many days or hours it will take you to finish.
you also have to take into consideration that cheaper will work for you. you're clients will save more and so give you more jobs in the future, even referring other jobs to you. if you start making your price competitive to other outfits (say like the last job you had) then what's to stop them from going with your past employer? be fair, don't be greedy.