If you want hit a new PR, you have to focus on performing more reps with as similar a load to your working weight as possible. Here's one way to do it.

4+2 Clusters

The 4 + 2 cluster method is an effective tool for performing more reps than you normally "could" with a heavy load.

1. Put your 5-rep max on the bar for a big movement (bench press, press, deadlift, squat, weighted pull-ups), but perform only 4 reps, leaving some juice in the tank.
2. Rack or put down the weight and rest for only about 10 seconds.
3. Un-rack the weight and do as many reps as possible. This usually results in another 2 reps. Now, you've just done 6 reps with your 5-rep max.

This exposes your body to more sub-maximal efforts under high work load, and the cumulative effect is nothing but positive for lifters after strength gains.