Oven Bake

There are two benefits of the low temperature oven bake - to undo any bending in the motherboard, and to remove moisture from the motherboard.
Flexing occurs due to the design of the original x-clamps and also because two of the stand-offs underneath the motherboard are usually too high. The moisture is only a problem if you are planning to do a reflow later.

This really is a very simple process but for some people it is just nice to have a guide for reassurance.

Ideally, the oven bake should be done at 60-70C (140-160F) without any insulation. Your oven may have a minimum temperature above the ideal temperature. In that case, the oven bake process can still be done but it would be best to provide protection for the more sensitive components. I have personally done a 90 minute oven bake at 80C/ 175F with no insulation and the board looks and works fine but that one did smell strange - I would not do that again. I appreciate that there are experienced members of the forum that routinely use this temperature.

Above 90C/ 195F and even with minor insulation, you are getting worryingly close to the thermal limits of the motherboard (the capacitors are rated to a maximum of 105C/ 220F, for example). In that case you will have to securely prop the oven door open to allow the excess heat to get out - all the way through the process.

Bare board: Make four balls of foil, squash them to the same height then use them as supports to hold the board off the oven tray. Just make sure none of the balls are resting on the ram chips.

Minor insulation (suitable for providing protection for temperatures up to 90C/ 195F): You will need foil and tape (masking tape, electrical tape, duct-tape). I have used masking tape twice and it has never left a sticky residue. Wrap the capacitors and plug sockets in foil and seal it with tape. The underside of the board does not require insulation at these relatively low temperatures. Make four balls of foil, squash them to the same height then use them as supports to hold the board off the oven tray. Just make sure none of the balls are resting on the ram chips.

Timings
There is no right or wrong timings for this low temperature oven bake. It should be at least 90 minutes but you can bake it for as long as you want.
There are many factors which decide how long the board needs to stay in the oven, for example:
How hot you have the oven
Whether you are trying to get as much moisture out as possible, to do a reflow afterward. (should be at least 8 hours)
The typical humidity of the country in which the 360 has been used.
The age of the 360.

To simply unflex the board and remove a reasonable amount of moisture, typical timings in the UK at 60C/ 140F would be 2 hours for a one year old board, 6 hours for a 4 year old board. In a tropical country, typical timings at 60C/140F would be 3 hours for a one year old board, 7 hours for a 4 year old board.

When the time is over
You do NOT want it to cool down quickly - you could potentially cause thermal shock damage to the board or more likely just burn your fingers. Turn off the oven, leave it to cool down for 15 minutes. Take the oven rack out using gloves and let it cool down for another 15 minutes. It should then definitely be cool enough for you to pick up and remove any insulation you may have used.

PM Weasel on the forums or try to find me on the Xbox-experts irc channel if you think anything needs to be amended.