My favorite way to tighten dreadlocks is with the crochet hook. They can be bought at most craft stores for about $3. It takes a little longer to tighten them with crochet hooks, but they mature faster and can be controlled much more.
Palm rolling is also an excellent way to tighten, and can be done anywhere at any time.
Dreadlocks love swimming in salt water. If you live by an ocean (one that is suitable for swimming), taking a frequent dip will do wonders for dreads. Palm rolling them while wet with salt water will help even more. You can also put diluted sea salt in a spray bottle and spray it in your hair. This will essentially have the same effect.