Soak them in a tub of water with a generous amount of regular table salt. Use at least a 1/2 cup salt in one of those plastic tubs that are at least 9 x 13. You can gently weigh them down to keep them immersed in the salt water, I never have though, I just periodically turn them. I usually soak mine for at least 2 hours, then let wicks dry and whoa, this has never failed me yet. The inexpensive candles from the Dollar Store for 3 for a Dollar, now not only are dripless but also last a long time cause they tend to burn slower as well.
One way to make your candles last longer is to make sure that they are level when you burn them. Sometimes the bottoms of cheap candles are rounded. Another thing that you can do is to make sure that you extinguish them correctly. Never blow out a candle! The wax gets blown down one side and then it will not burn evenly. Use a large spoon upside down or a special candle snuffer to gently snuff out the candle. Don't put candles near open windows, fans or vents either. Keep wicks trimmed and of course candles that have been kept in the freezer (sealed in wrap or foil) do burn longer.