Always cast your first tricks on the tank, even if they don't. You'll be delaying your onset of threat by 6 or 10 seconds, depending on whether you have the glyph.
Vanish is your friend when it comes to threat, and is actually a minor cooldown for assassination (thanks to overkill). Use Vanish 20 seconds after you start fighting, and then every 3 minutes after that. Threat will never be an issue.
Here are a couple of macros that might help out with making Tricks easier to use effectively.
The first one will let you set your focus via mouseover on party or raid frames. Keybind that macro, hover over another party member, and press it - voila, your focus is now that player.
/run local p="player" for i=1,40 do if(select(11,UnitBuff(p,i))==59628)then CancelUnitBuff(p,i)end end
/cast [@focus,help,nodead][@targettarget] Tricks of the Trade
The second one looks more complicated than it is. On the first cast, it will cast tricks on your focus or target of target if that person is dead (or you have an enemy focused for some reason). Press it a second time and it will cancel your threat transfer to that person. It's useful if you're casting tricks on a non-rogue player who would otherwise rush off the threat charts and get stomped in the face.