This is a monster of a drink but the best way to drink G&T, though you will find that every other G&T will taste peally-wally in comparison. Also, this really needs Blackwood's Gin but something with loads of fresh botanticals, like Hendrick's will work nicely.
In a large glass place 3-4 ices cubes and a good slice of lemon. Pour in a very large measure of gin and top up with the same quantity of tonic. Enjoy on your back lawn, whilst hosting a barbie - really takes the edge off dealing with other people's kids.