Here are some suggestions from an article by Charles T. Ridgely
"What Comes After the 5s?
Once you finish the 5s, you have choices on what to do during the two final weeks of your HST cycle. Certainly, if at this point you are feeling overtrained or are injured, you may wisely choose to terminate the cycle and go into a nice, rehabilitative SD. On the other hand, if you?re still raring to go, then you can do one of the following.
By far the most popular, and likely most beneficial, approach is to use your 2RM for negative, or eccentric, repetition training. With negatives, you generally have a partner help you lift the weight and then you lower the weight under control. You may choose to perform five of these negative reps or you may choose to perform two concentric, or positive, reps on your own followed by three negative reps where your partner helps you lift the weight. The choice is really yours so long as you can perform the negatives without injuring yourself.
Another popular approach is to perform drop sets in place of negatives. With drop sets, you generally use a much lighter weight than your 5RM, such as your 15RM weight, and you do the drop set as quickly after your 5RM set as you can. You can continue working out with your 5RM weights at the same volume you?ve been using and then do additional drop sets; or, you can use less volume with your 5RM weight and add drop sets.
Alternatively, you can continue to increment your weights on up to your 4RM or your 3RM. You can also add drop sets to these workouts. The only caveat is to watch out for failure. With such few reps, failure can come on rather quickly.
Of course, if you don?t want to use any of above-discussed methods, you can always continue working with your 5RM weights. Choosing this approach is beneficial because the 5RM weights are heavy enough to remain productive for two more weeks."
I have also seen logs from people that once they reached their pre-determined 5RM they continued with the 5's but added 5lbs each work out until they could no longer get the reps. The final succesful weight was what they used as their 5RM for the next cycle.
There are lots of options. Some of them are going to depend on whether you train alone or with a partner. If you are trainging alone it is probably safest to stay away from failure, it also make the negative cycle impossible.