I've harvested yeast that I pitched into a batch about a week later with no problem. Of course, yeast washing, proper storage, then starter proofing is the best way. When you rack your beer of primary, you can take some of the yeast out by either swishing the last bit of beer around and then continuing the siphon into a carefully sanitized jar or bottle. You can also get the yeast in suspension by pouring in some sanitary water and doing the same. Refridgerate the yeast to keep it viable. I don't like pitching harvested yeast until I taste the previous beer for contamination.. There's nothing worse than one bad batch except for two.