Sally's Beauty Shops sell a wig shampoo and conditioner in one. Swish, but don't run under running water. Place the piece in a towel, fold over the sides, and press to get most of the water out. Don't twist or scrunch it, causes tangles. Place on a wire head form to allow drying. Comb and brush gentle. Use a silicone based leave-in conditioner.
If cared for a synthetic wig will last 6 months at least, if cared for. Wash only once a month or so. And get the best you can afford. If you change styles a lot, buy cheap is fine, but remember you get what you pay for. The least I ever spent was $30 from a costume shop.
And use a wide tooth comb or wig brush, not a regular one. You will strech and break the hair. A conditioner will help to keep the hairs from geeting brittle. And make sure it has a sunscreen. That is what kills a wig quickly.
Janet