Pumping technique from my experience varies significantly with board type and body weight(65kg). I'll give you some examples of what I do on my different boards.

Hypersonic 105: Has extreme 'grip' to the water meaning you can pump the c@#p out of it and load up the fin to you hearts extent and it won't spinout. With this board I use massive, violent, unhooked pumps, slightly off the wind, front foot in between front strap and mast and really loading the fin when you have a bit of board speed. Typically I use a big sail with this board (8.5m) so this technique works well.

Maui Project (105L freestyle/wave type board): For this board I stay hooked in, bear off the wind a bit, front foot at the mast area and 'pump' it onto the plane by sheeting in and out quickly. I find this technique really effective, whereas a violent un-sheeted pump as on the hyper really gets nowhere on this type of board. Note because you don't have the large fin and board 'grip' you really want to be directing your energy along the axis of the board, whereas with the hyper it's not as critical as the loading the fin will translate into some forward drive.

RRD wavecult (75L): Not really conducing to pumping but I use the same technique as with the Maui Project.

Also use wind swell to your advantage. Even small wind driven swells on flatwater lakes can provide that added little bit of zip needed to get you onto the plane. And bear off the wind a bit, not too much, there is a sweet spot.

Like I said, pumping really does vary significantly from board/person alike but those are the two I use. They may be of some use to you :)

I pump hooked in and in the straps, I'm on wave gear. If I'm very clost to planing, it's just a matter of moving the back hand in and out fairly fast, but not very far, don't want to oversheet, Also use my back foot, if you have a flexy wave fin, that can also be pumped to advantage, bouncing the board out of the water a bit can also help.
As with a lot of windsurfing rechnique, it's a subtle progression. You can start of with big slow exagerated movements and as your speed increases, speed them up and make them smaller. I think part of the trick is, moving your weight and rig back as you speed up. Move back too fast and the tail sinks, move too slow and your not getting rid of wetted area fast enough,

I sometimes find getting the sail through the bedroom door a problem.

Thanks for the replies.

I tried it today, wasn't quite enough wind to get onto the plane, but pumping at least didn't slow me down this time

Interesting what Decrepit said about wetted surface area, I always thought that a flat board is a fast board, obviously there's more to it than I thought!