Resizing for big prints?

Just about to send some digital pics to be printed 20x16 or maybe even 28x20 but at 300 ppi the image size is quite a bit smaller than that.

I have used PS CS2 and increased the image size 10% each time, until the correct size is achieved, as I've heard that this method you get better results than doing it in one big bang.

Is this the best way of doing it or should I just leave the original image as is, as converted by Lightroom for example?

What does everyone else do?

Leave it as its native resolution and let the lab worry about it - they're specialists. The 10% thing is a decade old and isn't true any more - try it and see.