When you're freelancing, there will be natural droughts. And when they happen, I do fun stuff without feeling guilty (travel, personal projects, nothing). So take advantage of the droughts, but don't plan them.
I've been doing what tptacek calls 'staff augmentation' (i.e. drop me in any project that's in trouble and I'll make sure it goes faster/makes it deadline) and in 5 years I've never had more than a six week drought and even that was only because I didn't start looking until two weeks into that drought.
Learn to scan entire code bases (+docs/wikis) to get a solid high-level overview of the entire system and fairly in-depth view of the parts under active development in at most a few days. Basically cram like you would for a last minute university exam.
Blend in fully with current culture and process. If you choose not to, make sure you know the costs of change and do so only if you can decisively implement the change and have it deliver a net positive effect during your stay (hardest skill of all.)
Quickly establish trust in your technical competence without being perceived as showing off. Usually jumping in to help fix bugs, giving colleagues 'aha-moments' and identifying (elusively obvious) fundamentals work well for me.
Grab every opportunity you can to figure out what someone does and show them you're a good guy/girl, whether at lunch, in the elevator or even just in the hallway I call it 'collecting pointers into the organization.'
TL;DR Work your ass off at the start to consciously do in a few weeks what normally happens by osmosis in a couple of months.
I can't just work n months of the year and keep happy clients, but I've done 30 months straight for one client. That means I can comfortably take 6 months out now, over the summer months when the weather's better and the days are longer. We can book a family holiday; my wife needs to give 8 weeks notice before taking leave. I'm moving house and will be able to spend time doing the property up.
Personally, having the time out not at my choosing would be quite stressful and wouldn't allow me to plan house moves, holidays, etc. around it. I guess YMMV.
We used to have droughts as well.
Hence are launching this service to keep a track of good leads for freelancers round the year: http://topleads.co
One question - what do you do when you have many good leads, but have time to work on only one?