Let gravity do the work on getting the can to lay more forward. Originally, the waist buckle was used to hold the can in place. That's one of the reasons the buckle favors the right-hand side. What many people have adopted, especially for ocean diving, is a 2 buckle method. The can goes on first, followed by a loose buckle and then the waist buckle. The loose buckle and the waist strap buckle should be oriented in the same direction. This helps prevent the light from falling off when the waist strap is taken off. To allow the canister to move forward, release the buckle, move the can forward, turn on the light, move the can and buckle back up and slap the buckle down. This can all be done without ever touching the waist strap.
Another adjustment you can make, which a few others have mentioned, is to move the belt loop higher on the canister. This helps out a whole buch, especially on the smaller canisters like the Helios 9 and 4.5. It gives you a little more "reach" to be able to flip the switch.
Give these a shot and let us know how it works out.