You love the outfit, the shoes, and you would really like to love the bag, but it keeps slipping off of your shoulder and cramping your style. Before you stick rubber on your purse thereby cheapening it, or before purchasing an ergonomic strap- which can be bulky and won't work for several types of purses, consider a free and easy option- crossing the straps.
Put your purse straps over your shoulder as you normally would, with straps parallel to each other.
Allow the purse to hang like normal to get the most accurate strap length necessary for crossing.
Tuck the outside strap underneath the inside strap. Not only is this method effective, but it is subtle enough that you won't look fashionably impaired.
TIP
If your purse straps still slip to the point of complete and utter frustration, check out a product like the Strap Diva, a slip-proof strip that comes in various colours and keeps the purse in place.