Hopefully you took my advice from the previous post and bought good, strong thread. Take a length of thread -- I find an arm's length sews 2-3 buttons -- and double-thread your needle, that is, tie the two ends of the thread in a knot together.

Now push the needle through the wrong side of the fabric and through one of the button's holes, and back down through another hole and through the fabric. Before pulling the thread tight, however, pass your needle between the two threads still in back (i.e. pass your needle just under the knot). This will anchor the knot firmly in place and keep it from pulling out even if the fabric wears and the hole becomes bigger. Double strong thread is unlikely to snap under the usual pressures of daily wear and even under stress is surprisingly tenacious. Finish sewing the button on by passing the thread four or five times through each pair of holes.
*IMPATIENT TIP! If you are strapped for time or finger mobility, you can use your sewing machine to sew on a button (it's less strong but fast and you can pass the thread just about as many times as you like). You'll need to drop your feeder dogs all the way -- try setting stitch length to 0. I usually take off my presser foot as well and use the presser "leg" to hold the button in place. Set your machine to a zigzag stitch. Turn the wheel to do the first few stitches, making sure the stitch width is correct for the spacing of the button holes. Once you are confident that all spacing is correct, you can run the machine electrically (although it's still better to go slowly, and technically you can do this entirely by turning the wheel if you've got the wrist for it). When you've sewn several stitches, you can cut the thread and move on to the next button.
WARNING! This can be dangerous if not done right! If the needle strikes a hard button at a high speed (i.e. during electric sewing), either one or both may break and send sharp debris flying toward your face! Always exercise common sense and prudence when doing anything that may involve sharp debris flying toward your face. If you're nervous about it, safety goggles might not be a bad idea.