Personally I have a page on one of my hotbars with all of my poisons as well as my mainhand and 2h weapons so I can click the tabs to apply the poisons then click the weapons to swap and then add poisons to those as well.
You could even have /switch keys on this bar for juggling weapons from your bag into weapon slots to get poisons applied to 6-9 weapons fairly quickly without any need for opening your inventory.
As Noah said, poisons are not really needed when leveling. The DoT poison can add a bit of damage, but unless you have a bunch of weapons you have to wait 10 seconds to reapply after each mob, and that sucks while leveling. Sheelba also has the right idea about weapon swaps for later on. I use /switch macros that are named after a poison type so I know exactly which one to put on a given weapon at any time. Though once you play the character for a bit the names become irrelevant as you just memorize the key binds, they did certainly help me keep track of poison types easier than Live was with drag and drop.
If you do decide to use envenom while leveling, just throw the poisons on a hot bar and click them (No need for macros while leveling). They default to your mainhand, then offhand if the mainhand has poison on it already. Then just switch over to your two hand slot, and click another poison.