I do a similar process to rover. There are plenty of threads about it so please use the SEARCH button.
After bottling all of the beer I add 500ml of boiled, cooled water into the fermenter. I give it a swirl and pour it into a sanitized jar. Sanitation is really key here, if you get lazy you'll screw your next batch, or 5. After the sanitized jar with your yeast has been sitting on the bench for 15 minutes or so, most of the trub (bad) will have settled on the bottom of the jar while the yeasty goodness remains in suspension in the water. Decant this yeasty goodness into another clean, sanitized jar - seal it and stick it in the fridge. If you plan on keeping it longer than a couple of weeks than I'd suggest repeating the process every couple of days. I usually do it 3-4 times. Eventually the yeast settles and the remaining water is left clear.
Yeast washing 101. I feel I should reiterate SANITATION is really really important, clean everything with whatever brewer's detergent you use then rinse all that detergent off with hot water then sanitize - I use iodophore which is really easy to use and rinse free. Just squirt it everywhere with an atomizer and leave it for a few minutes...easy.
