Burr veneers can be frustratingly difficult to flatten. I have found the following to work fairly well.
Start by wetting them fairly well (well enough that they don't crack when flattened but note wetter), place between several sheets of newspaper and then sandwich that with MDF or chipboard which in turn can be weighted down or pressed if you have a press. Leave a few hours then change the newspaper and leave again weighted down for several more hours.
Remove the veneer and leave it on a flat surface to dry properly. If it stays flat, great, other wise you will need to make up a flattening mixture and repeat.
A formula for a flattening solution is
* Four parts water;
* Two parts glycerine;
* One part meths and
* Two parts PVA.
Glycerine is expensive if you buy it a chemists and at a pinch you can use just a glue size PVA (thinned with water) but when using this gunk you need to cover the veneer with thin plastic to stop newsprint sticking. The thin decorators' dust sheet stuff works OK.
If you apply the veneer before it's dry, it will crack as it dries and that can really spoil your day, so don't be tempted to do this.
If this doesn't take your fancy try some of the ideas here
http://www.joewoodworker.com/veneering/flattening.htm
