You can't. When the shower is heating colder water - like in winter - it needs a longer time to do it. It gets the extra time from reducing the speed of water through it, so the pressure drops.
It's normal.
Your only answer would be to fit a more powerful shower.
Colin
December 2007
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.