Yeah, just ask! When you see a customer squirming inside, they are probably dealing with the feelings of guilt and responsibility which they know will follow the purchase. If you ask for the sale, that guilt is partially transferred to you, since it becomes your idea. You become the pascal lamb for their actions. Guilt is mostly irrational, but among its core properties is that it can be easily transferred to other people. All you need to do is volunteer to be the recipient. See also: religion.
The best darned art salesman I know is also one of the bluntest about asking for sales. If a customer is looking at a piece, he asks if they would like to buy it in a friendly, matter of fact voice. When I first started showing at his gallery I was shocked by his directness, which told me something about myself. We artists have to get over the notion that sales must involve great delicacy, artful language, and subtle schmoozing. We just need to ask in a friendly way. We need to be a kind of partner in the decision to buy, rather than an aggressor. Most customers appreciate directness and candor about your intentions, and quite a few will become defensive if they sense they are being subtly manipulated. Anyway, that salesman puts more food on my table than anybody else including me.
Don't neglect physical presentation of the art and yourself, and there are dozens of things you can do to your gallery or booth to encourage sales. But above all else the customer must be allowed to feel that it's OK for them to buy the piece.