I should start to get serious about marking tanks...

TeriyakiTanker: You are ridiculous, obviously most players will never 3 mark even one tank.   As to the original poster, for me at my skill level food is an absolute must when marking a tank as high as I can.  Drop the fire extinguisher if you can, it helps to run the directive and have a crew skilled out with the skill.  Assistance damage is key to marking, but remember its not just spotting.  If you can track a tank thats potentially a huge amount of assistance damage, now imaging if you can manage that two or three times in a game.  It really adds up. Also consider taking the Designated Target perk along with the directive.  When I did this the tank would stay lit for 14 seconds instead of 10 seconds after I wasn't spotting it anymore.  I felt this was a huge advantage when marking my Leopard PTA.  Not only do you get the benefit of the extra 4 seconds of spotting but sometimes the tank doesn't realize they are still lit and will move back into a bush or poke again.