1.] Use WARM water for natural fabrics (such as cotton)
2.] Use COLD water for synthetic fabrics (such as polyester)
3.] Polyester (and poly blend) clothes look better if they are hung to dry. I don't really know why, they just do from my experience.
4.] Make sure the sink is clean before you wash your clothes in it! (I know, this seems obvious...)
5.] COLD water will keep colors from bleeding (if the fabric is colorfast).
6.] Only wash one garment at a time. Even if the colors are similar. It's better to be safe than sorry.
7.] Dissolve detergent in warm/hot water first, then add to your rinse water.
8.] Don't agitate the fabric too much (or at all if you can help it). Usually I sort of just pat and carefully "knead" (sorry, I can't think of a better way to describe it) the fabric. For dresses I will try to flatten it in the tub and gently "pat" the detergent in. If it's a sturdier fabric, I agitate the fabric just a bit.
9.] Rinse vintage garments in cool water. Make sure to rinse them well. You don't want any detergent residue!
10.] If in doubt LEAVE IT TO THE PROFESSIONALS. I am by no means a professional washer/restorer of vintage clothing, so the more delicate items I let air out instead of risking damaging it forever.
---- More tips on washing and caring for your vintage soon!
Image source: Laundry by Nudonudo