Adding weight is the best form of progressive overload, but not for lateral raises. Sure, you'll be able to bump the weights up a little bit when you first start doing them, but most people plateau quickly and subsequent weight increases just mean using worse form. It then becomes hard to measure progress. Did you get stronger or did you just cheat more?

Instead, keep the weight for lateral raises lighter and focus on maintaining good form with your arms reasonably straight and body English kept to a minimum. If you use some momentum on the last couple reps of a set, that's one thing, but if you're heave-ho'ing from the get-go, the weight is too heavy.

Instead of chasing weight increases, try to increase the reps or the volume, adding in a few sets here and there or doing them an extra day per week. Or just do this.

Lateral Raise: Countdown Method

Start by choosing a set number of reps, usually somewhere between 4 and 6. However many reps you started with, you'll immediately follow that up with an isometric hold, held for an equivalent number of seconds. Then you begin the countdown, all the way down to 1 rep.

Let's say you chose to start with 5 reps. The set would then look like this:

5 reps
5-second hold
4 reps
4-second hold
3 reps
3-second hold
2 reps
2-second hold
1 rep
1-second hold

Related:  Big Scary Delts

Related:  The V-Taper Workout