I also use the stretch relief pliers from Breathing Color.
There are a couple of things I do you might want to try.
1. I will tack down the first side to create tension, then go to the opposite side using the stretch relief pliers and staple at least 4 times. I also staple about 1/2" apart. Once I have this side down, I go back to the first side that was only tacked down, remove the staples and use the stretch relief pliers and reattach this side. This IMO gives you a bit more tension.
2. Before I continue down one of the long sides, I will add 4 staples to the top of the short sides, by the corners. I use the pliers to do this. I believe this will help pull out the ripples that you are talking about.
Once done with this, I will go back and finish one long side then the other. When stapling, I start from the middle and work out to the edges, working in both directions.
When I get ready to start the short sides, I will again start in the middle and work to the corners. I might have to pull out the temp. staples I added at first.
Here is a link to a review I wrote about the stretch relief pliers:http://photosofarkansas.com/category/reviews-of-equipment/printing-framing/
In this review I describe my technique in a bit more detail.