I am by no means a veteran, as I have only been self-employed for one year; however, this is how we do it my transactional law firm :
Flat fees for simple agreements when that agreement is the only thing my client wants- but only when I would earn more money by charging a reasonable flat fee than I would by charging hourly fees. Hourly fees for anything involving actual research. Hourly fees even for simple agreements if the client is on retainer and I am expecting to form a long-term relationship with them.
For fixed fees and hourly rates alike, I consider the size of the potential client (i.e. small sole proprietorship vs. large corporation (and the different sub-categories of such corporations based on their known or estimated net worth)) as that tends to show how complex the contract is going to be and what the dollar amount of a potential malpractice claim against me is going to be. I also look at what would make it worth my while to actually help the client. For example, if someone comes to my office asking me to draft a simple easement agreement, I will not touch it for under $800 even though it will likely take me one hour of actual work, because I hate property law and I do not have any associates at this point to do the grunt work for me.
There are two types of clients I generally encounter: those who want the cheapest price possible and those who want high quality work and do not mind paying for that quality. At this point in my practice, I can be a little more picky and have little patience for the former.