Once you reach Level 30, obtain Superior (purple) or high-end (orange) gear from vendors that have at least one empty mod slot. It is also very helpful if they already have an "Advanced Performance Mod" to boost the "Increase Kill XP" stat. Then, go to a Recalibration station (for example, in the Post Office at the Base of Operations), and keep re-rolling the stats on all the gear until it gives a boost to the "Increase Kill XP" stat. Go to Dark Zone 1, 2, or 3 with a team and kill as many enemies as possible to increase your Dark Zone level much quicker. If you get the "Increase Kill XP" stat to around 100%, you will increase your Dark Zone level twice as fast as normal.