Okay, being a PLS, I'll let you in on my trade secrets, 4 years of college and 25 years experience.

You guys are kind of freaking me out now. Advice is flying all over the place, some of which if good and some not so good.
First off, GPS is great in the open, not worth a darn in the woods. Like somone else said, the cosumer grade stuff will get you close, but not close enough and it doesn't work as well in the trees either.
The lasers aren't a bad idea, but how do you know the one laser is shooting straight up in the air? And I don't mean sorta close, but exactly. A carpteners level is not good enough to determine if it is plumb.
Yes, distances are measured horizontally, as if the world is flat.
The real trade secret is in the math, which is all done by hand held data collectors now. For example take a lot in town. Imagine a line 100' long with corners on both ends but a large tree in the middle. You set a new point at a random location about halfway between the two corners but roughly 30' off the line. From this new point you can see both corners, you measure the angle between the two corners and the distances from your new point to the two corners. You can then do the math and set points on the boundary line at any location you want from this random point. The math in the past was pretty time consuming, even with calculators, but with hand held computers, its very fast. The cost of the equimpment I just described would be around $10,000. You need a total station(a modern transit), which measures the distances electronically, prisims, which are refelectors, and a hand held computer. Then you need the knowledge in how to use it. In my above example if the line is say 3000' long, it takes multiple random points and alot more work to calculate points on the line.
The best advice from above is to hire a proffesional to do it. It could be done with a transit, but the problem is in the knowledge on how to use it. Someone described running a random line above, this would also work if done correctly. In one of my above posts, I described wiggiling in on the line with a transit, this would be the best method. The problem with doing it yourself is how do you know you are correct? Thats something you gain with experience.
The bottom line is I have perfromed hundreds of boundary surveys, several of which are on hilly wooded ground. I have located fences along boundaries that are one half mile apart(a quarter section line). It is not unusual to see these fences several feet off. I'm sure alot of these fences were put in using some of the above described methods by do it yourselfers. As a land owner, you have the right to do this yourself, it up to you to determine your comfort level. If the land is not very vaulable, and your adjoiner is O.K. with you doing it yourself, have at it. Otherwise hire a proffesional.