One of my favorite tunneling Tactics is the super secret Camo entrance.
Put one camo entrance almost directly behind a normal entrance. Pop a cave in on the normal entrance early on, then pop up from the Camo that the MI have probably retreated near. Not perfect, but works quite often as the MI's first instinct is to move backwards, but never so far that they are out of morita range.
Another one I've used to great Success is mobile terrain. This works GREAT against Chickenhawks. After deployment, place a tunnel entrance right in front of a Chickenhawk. EVEN if it is readied, it can't jump and fire, so if you are a higher PL, it must move on its first action and only get 1 shot off first turn. Doesn't sound like much, but saving yourself from just 1 action of its shooting can be a big help.
Also, If the enemy chooses to go first, this works even better...
You can use them for defense! Tunnel Tunnel Tunnel all touching each other RIGHT in front of your main advance! That's a 12" long 4" tall WALL in front of your advance. This works really well for protecting things like Hoppers or firefries if you find yourself wanting to choose defend tactics to avoid possibly getting an Encounter mission. You move forward next turn, and your back line then performs a ready action to collapse the tunnels and they go back to your pool. Then, keep one in the center of the table and you can tunnel in ANY direction from it.
Ok, I could go on and on.... I think I'll just go back to work on my tactics articles, I've been slacking a bit.