As I mentioned earlier, the C5 oil drain plug is at the front of the oil pan, so the nose of the car should be lower than the rear in order for the oil to drain forward. Having the rear of the C5 elevated about 2-4 inches higher than the rear will do nicely to help evacuate the old oil. On the C6, the drain plug is next to the oil filter, so the vehicle should be level for changing the oil, rather than having the rear slightly elevated a la C5.
Before removing the drain plug, however, open the hood and remove the oil filler cap and pull the dipstick up a bit so that it's not fully seated. This helps to relieve pressure and enables the oil to drain a bit faster. I also suggest that the engine oil be warm when doing the oil change; warm oil is thinner and flows more quickly than cold oil.
Next, it's time to position the drain pan underneath and slightly forward of the drain plug. Use your 15mm box wrench or socket and ratchet to loosen and, ultimately, remove the drain plug. Give it at least 15 minutes for the oil to drain. When the flow has slowed down to a thin trickle (or better yet, intermittent drops), you can replace the drain plug; 15 lb-ft is the factory torque spec. I use the "feel" method, however. That is, I tighten it until it feels tight enough to me. Be sure not to over-tighten the plug-it's a bear to remove at the factory spec, so it may well be impossible to get it out without ruining it if you go too tight. Now you're ready to move the drain pan under the oil filter and remove it.
A word about magnetic drain plugs is in order here. For about $5.00 you can replace the stock drain plug with an aftermarket magnetic drain plug available from Mid America Motorworks and other suppliers. In the overall scheme of things, $5 is very cheap insurance to help safeguard your engine from possible damage that can be caused by stray, minute steel or iron particles that may be suspended in your oil circulatory system. At the very least, five bucks will buy you a lot of peace of mind.
If you use K&N Oil Filters, like I do, here's a little tip to help keep things neater. The K&N filters have a 1-inch nut welded to the bottom of the filter, which makes for easy removal using a 1-inch open-end wrench. I loosen the filter about 1/2 turn, then slide a plastic bag over the filter and unscrew it the rest of the way by hand. As the filter comes off, the plastic bag catches it and the resultant dripping oil, thus preventing it from getting on the garage floor or driveway surface. Ostensibly, this trick will work with other brands of filters as well, once they're loose enough to be spun off by hand.
And, if you really want to get some extra engine protection, you might consider investing in an oil filter magnet for a few dollars more. Mid America Motorworks has a nice disc-shaped unit that grabs onto the bottom of an AC/Delco filter or other smooth-bottomed filter and traps and holds any stray ferrous particles that enter the filter from the lubrication stream. This disc-shaped filter magnet doesn't work with the K&N filters due to the 1-inch removal nut abutment I mentioned earlier, so I use a cylindrical FilterMag wrap-around oil filter magnet available from www.filtermag.com.
GM specifies that you must use oil that meets or exceeds General Motors standard GM4718M (synthetic 5W30) in order to maintain your new factory warranty. However, not all synthetic oils meet this standard. So be sure to purchase oil that meets or exceeds this standard. GM uses Mobil 1 5W30 as the factory fill on the assembly line at Bowling Green. I use Royal Purple 5W30 synthetic, but that's my personal preference. You can use any brand you wish as long as it meets or exceeds the GM standard.