There are TONS of methods, but one of the simplest (physical activity) have already been mentioned. One method the military uses is the "Sgt. in your face"... basically have someone scream obscenities (or at least scream) in your ear as you shoot. Of course this should be controlled as well (for safety) and likely wouldn't fly on many ranges. We also shot in full gear and body armor and would do fire and movement courses in 100 degree weather... that'll do it too.
If there's an outdoor range, you could make a point to go on the nastiest, hottest, coldest, rainy, humid, miserable days that they'll be open for. Misery makes stress.
When I went through police academy, the firearms instructors actually did a very good job of simulating stress during the combat course. It really rattled and shook up alot of the guys. The instructors followed the shooter around the entire time ridiculing them. We were forced to find our rounds and shells in a pile of spent casings, all while someone is screaming in our ear. After finding the rounds, we had to run, seek cover, and engage. You had to have 4 of 5 hits on 10 inch plates from the 15 yard line, and if you didn't then you had to go back to the beginning, do pushups and situps, and start all over (Some people did this 3 or 4 times and they got pulled from the combat course for extra coaching). After the first target, you continued through more of the course (didn't have to start over if you goofed after the first target though). None of the shots were overly difficult... there were a few 10 yard shoots on a slow moving torso target, most of it was from the 7 yard line. The stress really added to the challenge though. Only a couple of us passed on the first try.
One of the easiest ways of all to induce stress is a timed course fire with a gentleman's wager riding on it. It's not in your face stress, but it's time stress and you have something riding on it. The clock, if you make the time a challenge to you, can mess with your head as quick as anything. Simply go to the range with a buddy and have him time you while you conduct a course of fire. Do it after you've already practiced some so you can both agree on doable, but challenging, time limits and hit percentages. The wager doesn't have to be big, but it has to at set you back at least a little. It could be you buying the beer for the "after the range relax" time, paying for the next range trip, or even just washing your partners truck for him. Of course, you return the favor for him. The beauty of this method is that all other tactics may not be doable at your range, but this one can be done anywhere. It's not the most stressful environment, but it does add some performance anxiety to the mix and that's better than nothing.