I like the suggestion to put the bike in a trainer or get someone to hold it for you while you practice. If neither of these is possible, you could always lean one hand on a wall/fence/railing/whatever and work on getting the other hand into the swing of getting the bottle in and out - try to do as much of it as you can without looking. Like, just take little glances at your bottle/cage so you know where your hand is going. It's much easier to ride in a straight line when you are looking where you are going!