Taxi is probably the easiest way from LHR. If you are visiting a company or when you find a hotel, I would suggest you ask them to book you a mini-cab company to come and pick you up - much cheaper than the lovely London Black Cabs and always good to get off a flight to find someone waiting to drive you away. Mini-cab would probably be around £35.

From central London there are direct trains from Waterloo station if you are travelling light - they are a bit slow but you get there in the end - you get the pleasure of looking at the breat British countryside in Feltham and Staines .

Alternatively I would suggest you take a train from Paddington to somewhere like Maidenhead and get a taxi if you are in a hurry or staying near Paddington. Cabs from central London would need large expense account or new mortgage....

Hope that helps...