Sanitize a stubbie.
When you have filled your last bottle with beer (and you don't think you can get another from the fermenter), pickup up the fermenter and do the Twist, you know, like you did last summer. Then twist again, because twisting time is here.
Put your fermenter down because you look like grandpa at a party doing that old school dancing.
Now fill your stubbie (I use a 300ml PET coke bottle) with the milky crud that comes out the tap. Cap it and mark it with yeast type and brew type, and fridge it.
I've found zero issues up to the 3rd generation. Possible issues after this, and definate issues past 5 generations.
The day you are brewing, take the correct yeast stubbie from the fridge and let it come up to room temp (if it's a 35 degree day, crack it when it's about 12 degrees). Crack it, pour off the clear beer at the top and taste it ... if good, pour it in. Put it this way - if all the bottles you bottled are fine and tasty then you can be almost certain that the bottled trub under the same conditions is also fine ... but still taste the clear beer on top, just to be sure. And no, don't drink it off the top of the stubbie!
Cost: $0.00c. Pitching rate: about right.
EDIT: in your big book of brews, keep track of the generations. With five yeast types and a few generations it's impossible to remember what's what without good yeast accounting. I chuck the trub at the 3rd generation nowadays - but remember that you can always bottle two bottles of trub when you're bottling. This will half the time to get to 3 generations - or maybe it'll be x squared? My brain hurts.