The only way to guarantee full time employ is to become a company SM. Personally, it is not my style - I like to pick and choose shows I work on. This of course means that I do not have employment 48 weeks in the year, instead have a far more seasonal career, filled with months of extream business, followed by months of pure nothingness. Now, I realise that that is not ideal for most people, but as I am one of those 'Jack of all trade' backstage techs who does anything and everything, I can usually fill the quiet months with a few odd lighting or sound gigs for events and bands, and failing that, do some temp type work.
The main thing you need to do is to get a good diary, a big wall planner and a good pair of marker pens if you are not going to be a company SM.
Then, as each job comes up, mark it in both. The time I did this, I coloured in the squares on the wall planner and only used black. Big mistake, as I had a second, evening gig that ran during the rehearsal period, and could not write it in or diferentiate between the two. My current way, which seems to work, is to rule a line in black for rehearsal period, red for performance period and both black and red for tech week.
I find that rehearsals are largely day time, and performances are largely night time and it is often possible to double book during the rehearsal period with performances over the top (after establishing that they did not do matines). Tech week however is not possible - so a fully booked season for me will have two lines on every day, a rehearsal and a performance, although I try and limit myself from doing that.
I suppose what I am trying to say is if you play your cards right, you can deal with periods of inactivity by sensible practices. A good habbit is to work out how much you need to live off of in a week. For me, that is AU$180 (including rent and bills that occur monthly and only covers bare essentials ie food and petrol etc). Now a standard week I may bring in (for example) AU$600 after tax (I have no idea what the actual figure is). Now what I do is take $200, and it goes into my expenses account. Next, I take another $250 and it goes into my 'dry period account'. The final $150 I put in my savings account. Now that means that I have enough after 10 weeks of work to live for just under 14 weeks without dipping into my savings. If I stick to my strict budget of $180 per week, my expenses account will also be in credit by $200, which is another week, or something nice to spoil myself. Now my period of inactivity usually contains the odd job, which also supplements. When I get a fair bit in the 'dry account' I transfer some to the savings account.
It is all a matter of how you want to live.
Mac
... PS. Another good trick, when people are booking in advance, get written, signed confirmation. Nothing worse than when they back out. Especially if you have had to knock back another job because of it. Do not make it pleasent to back out late either. I have a standard contract I get them to sign when booking me, which has a clause stating that should they cancel my services within 2 months of the job, a $120 booking fee is required, or renumeration for the money lost. Most people will understand if you explain it, although I lost three small gigs because of it. I know the person who picked up two of them though, and one of those the person cancelled on them two nights before.