A web-based ticketing system that you can expose to your clients helps immensely with keeping track of everything, especially if you give them permissions to file and manage tickets against their projects themselves. That way they can check status details, re-prioritize things, add new bugs/features, etc. without your direct intervention.
Personally, though, I've never gotten into a situation quite like that, even when I've had several projects for different clients running at once. Whenever I talk about upcoming work with a client, I also tell them when I expect to be able to finish it and, if I'm particularly busy at the time, when I'll be able to start. Once I do start on it, I ask questions as they come up (usually by email, but I will use the phone if it's something I need answered before I can progress further) and notify them of major points being completed or if I expect a delay.
By keeping clients consistently informed of enough details to suit their needs for assurance that progress is being made, I've managed to avoid being asked for status reports for over four years of freelancing, with the sole exception of cases where there is a non-negotiable standard "you must file weekly status reports" clause - and even in those cases, the vast majority of my status reports have simply consisted of "see communication from earlier in the week".