To make the lateral raise effective at recruiting the delts (and not just the traps) first focus on pushing the dumbbells AWAY, not on lifting them up. Try to bring the dumbbells as far to your sides as possible. They should only go up as a result of you pushing sideways. This minimizes trap recruitment.
Here are three alternative exercises to try:
1. The Backpack Raise
No, you won't be doing lateral raises while wearing a backpack (although that would likely work too) but with resistance bands looped around your shoulders to keep them down.
The traps get involved when the shoulders raise up instead of just rotate. The bands, by keeping the shoulders down, help you focus on the delts better.
To set up, you step on the inside of the band and hook the other end around the shoulder. Then do that on the other side with a second band.
The bands' position on the shoulder is important. You want to place it on the AC joint, not on the trap. If the band is on the trap it will actually increase the recruitment of the trap by creating a greater mind-muscle connection with that muscle and a reactive contraction because of the pressure.
You'll still need to focus on pushing the dumbbells away instead of lifting them up, but the bands will make that a lot easier.
2. The Handcuff Raise with a Mechanical Drop Set
For this one you'll use a short resistance band looped around your wrists, like handcuffs. Use a band with only a small amount of resistance; no need to go crazy here since you're only using it to shift the tension to the medial delts.
Choose dumbbells that are a bit lighter than what you'd normally use for 10 strict reps. Let's say a weight you could do 12-15 quality lateral raises with.
- The first step of the mechanical drop set is to do partial lateral raises with the band and dumbbells. Go as high as the band will allow, which should be around a third to a half of the way up. Do as many good reps as you can.
- Then immediately drop the band and do regular lateral raises with dumbbells only. Shoot for 8-10 reps.
- Then, drop the dumbbells and put the band back on and do partial reps (like in step one) with only the band.
Don't rest between each step of the mechanical drop set. If you want to set your medial delts on fire, this is the exercise for you!
3. The Incline Lateral Raise
This is the "less cool" option but one that I've been using for at least 15 years successfully with people who have dominant traps.
Sit down on an adjustable bench angled at around 30 degrees and do lateral raises from that position. Still focus on pushing the dumbbells out, not lifting them up. This greatly decreases trap activation, but you still have to focus on pushing the dumbbells far away from your side instead of up.