The easiest way to use yeast more than once is to dump the next batch of cooled wort onto the lees of a just finished brew. You just have to coordinate your kegging/bottling with brewing and do as much as you can in the way of not letting anything jump in. I use a drum tap and tube off my primary, so only the airlock is removed (while draining) until I am ready with the next batch. (Taste the outgoing beer just to make sure that it is not off in any way.) I use hop straining bags and grain strining bags when brewing, so I dont have much gunk other than yeast slurry at the bottom of my primary. I successfuly make three batches of ale on the one vial of yeast.
Now, to push the envelope and save yeast for use another time instead of back to back, I sanitize four beer bottles, bottle caps, a funnel and a measuring cup. Now at the end of kegging, I remove the lid, carefully scoop out four yeast dense scoops and fill the bottles (actually only about 3/4 full). Caps on and into the fridge. I lable them and give them a generation number. I don't plan on using any type more than four generations in my keeping (five in total). To revive: bring a bottle to room temp, (the yeast will have formed a cake on the bottom) prepare a starter flask, sanitize the bottle top areas with vodka, then uncap, sanitize again with a butane flame, pour a little of the beer off the top and taste to see if it's nasty, then if good, innoculate the starter. Ready to go in a few days. I have brought back WLP005 007 and 002 - nearly two years later!
Before pitching into a new brew, having sanitized the lip of the start flask, I do pour a small amout for tasting (again). Yeast slurry does not taste particularly good, but with some experience, you will have a clear sense of whether something really bad has gone wrong.
In this way you can make one vial go: 1:4:16:64:256. That's a lot of brewing and my fridge is not that big. Plus I like to keep three or four strains handy. So what I actually do is just keep four bottles of each in stock replacing as needed. When I hit re-use level four, I buy fresh. Four is just a number. I've seen ten and even 30 bandied around.
All of my regular beers are variations on pale ales. I would not suggest trying radically different styles from batch to batch as the yeast might mutate and rebel.
Together with homegrown hops and buying DME in bulk, my brewing is about as cheap as extract brewing gets. Give it a go. I was nervous to try at first but, so long as you keep a careful eye on sanitation, it's easy and saves a ton of cash.