There are actually several things you can do, if they are set triggers they can be very carefully polished to smooth the actions, be careful that you don't change geometry or polish through casehardened parts though. Springs can also be lightened, though this isn't all that good an idea unless you are sure about what your doing. A single trigger can also be adjusted to the point that it is as good as a set trigger, the best way to learn this trick is to buy one of the books on building longrifles, they generally give a good description of the geometry you need to accomplish. Bear in mind that this may involve redrilling the pivot hole, and or making a new trigger entirely if the original is too far out of line. This isn't too bad a job though and doesn't require any special tools other than what is probably available in you home repair kit though. The last thing I would do is to check the lock itself. The relationship between the sear and the notches in the tumbler are often to blame for a lot of rough trigger pulls. Basically this area is critical to a safe and easy let off and it's something that should only be touched by a professional gunsmith, or someone who has taken the time to study appropriate books on the subject. One thing that can help is to polish the arm on the sear, to reduce friction between the trigger bar and the sear.
