Building a set of respectable traps is simple but not easy. Here are two tried-and-true exercises to build strong, functional traps.
If you've ever done a scrape-the-rack press, this is the same idea but for traps. It's similar to shrugs on a Smith machine, but better.
Start by placing your toes in front of the bar so you're off balance and leaning against the rack. Get a big stretch at the bottom of every rep to maximize the ROM (a slight hip hinge is totally acceptable).
By sliding the bar against the rack, you create more resistance and isolate the traps far better than conventional barbell shrugs.
Use straps and go heavy for 2-4 sets of 15-20 reps. Hold for 15-20 seconds at the bottom after the final rep to really stretch the traps and maximize the growth stimulus.
Arguably the best exercise to develop traps. Few exercises are more effective (or functional) than just picking up something heavy and carrying it.
The trap bar offers more stability than using dumbbells since your hands are in a fixed position and you're carrying only one object.
Trap bar carries can be done as a stand-alone exercise for brute strength or as a great finisher to your back/pulling day. Either way, your traps can tolerate a lot of punishment, so use moderate to heavy loads for maximal benefit.