Maybe someone will find this useful. I was stumped for a while figuring out the best way to remove tension on the belts without breaking something.
Removal of bmw e46 alternator (mine was a 2002 325i 2.5L)-
First disconnect the battery in the trunk. Remove the ram-air duct held by 3 expanding rivets at the top of the radiator - don't lose them. Remove the intake/air filter housing - two clips hold it to the MAF sensor, 2 screws hold it to the driver's inner wheel well. Remove the two screws that hold the power steering reservoir in place - it can be moved aside without disconnecting the fluid lines. Disconnect the alternator positive cable and regulator sensor connector which are both on the back of the alternator. Disconnect the two sensors from the electric fan assembly so the fan can be removed. The fan assembly is held with a torx on one side and a plastic expanding rivet on the other - remove those and it slides up and out. You now have a space to get to the accessories at the front of the engine. Examine the sepentine belt route - there is an idler attached to the side of the alternator and a belt tensioner below the alternator. The belt tension must be relieved to remove the alternator. Removing the hub cover from the tensioner pulley exposes an 8mm hex recess. This must be torqued clockwise (facing the engine) to compress the tensioner spring and slack the belt. You wont be able to do this with a regular Allen wrench without it sliding off and breaking something. You'll need a 8mm hex on a socket and 2' breaker to apply enough torque. It will rotate about 60 degrees from its rest position and you should then be able to slip the belt off the alternator's drive and idler pulley. Remove the cover from the alternator idler pulley hub to expose the top alternator mount bolt which is 16mm - not that common. The idler will come off the alternator. Remove the second (bottom) alternator mount bolt - again 16mm. Slip off the air duct going to the bottom of the alternator fan shrould, and the alternator should lift out - if it binds, wiggle it until it comes out. Goes back together the same in reverse - take time to consider that everything is replaced properly.
If you're doing this, it's also a good idea to replace the accessory drive belt - having it break can be bad news. The accessory belt mounts the crank pulley behind the AC drive belt so the AC belt must be removed first. It's pretty easy and has much less tension than the accessory drive belt. I just pushed it's tensioner down with my hand, and jammed an Allen wrench into a little slot that opens up in the tensioner's base.