The main cause of the shower pressure dropping is when water is called upon in another part of the house.
Usually it's in short bursts however flushing the toilet can cause what seems to be an ever ending dribble at the shower or a scold from the hotwater as it continues to flow untempered. A good way to repair the problem of the shower pressure dropping when someone flushes the toilet, is to turn the faucet or tap at the back of the cistern down, so it doesn't allow the water to rush in so fast refilling the cistern. Thereby regulating the pressure in the shower and a not so noticeable drop in water pressure.
This remedy can be also applied to the washing machine and dishwasher. After all we don't really need the express delivery of water in these appliances.
Other causes of shower pressure drop are clogged shower roses. Sometimes a build up of calcification or rubbish escaping from the pipes can build up in the shower head. Simply unscrew the rose or head and clean out. rescrew back on, usually there is no need for plumbers tape here as there is no applied water pressure on the thread.
Water restrictors in shower heads and aerators can cause pressure drop also. These might be required by building regulations in your area so i can't advise you to take them off but some are easily adjusted to allow for an increased flow.