1. Put a few inches of water in the bathtub with a little soap in it, (dishwashing liquid or baby shampoo will work well).
2. Fill up the tube with a good amount of pressure. Not so much that you can't bend it, but you need to give the rubber a little stretch to find some pin-prick leaks.
3. Slowly inspect the tube in sections under the water and just above the water line, (you'll be rotating it as you check it over). The soap will foam up at the site of the leak.
4. Check the valve in the soapy water also. Sometimes grit will get into it and keep it from sealing.
5. *****INSPECT THE TIRE. Repeated flats often indicate a compromised casing. Usually not to the extent that it needs replacing, but a shard of glass or metal could be protruding through the rubber. Removal of the foreign offender is likely all that is needed. Use a cotton ball and run it around the inside of the tire. The fibers will hang up an any sharp objects sticking through the tire. You can also run your fingers around the inside of the tire, but the bloody digits just end up making it harder to get your wheel reassembled, after you've patched the tube.