The Orange Box

This one is 100% guaranteed that you get it within the first five tries if you do exactly as shown. Remember, Chamber 16 at the part where you can go into the wall.

*Note: Cheat codes disable the ability to unlock achievements so be sure all cheats are off*

Here is how I got mine. It's very easy to do (got on first try) and you don't have to hold down, etc. You just keep bouncing off the glass wall.
(Credit goes to nintenexprt)

I got this Achievement right at the beginning of Chapter 5, Testchamber 13. But it can easily be obtained anywhere else in the game with the same wall setup

Here is a video of myself on Testchamber 13

I find this one to be pretty tough, because I can never seem to get the portal placement perfect. Follow the included video and you should get it eventually. The video also shows you how to get the Terminal Velocity achievement.

No video here. But my simple way of getting this achievement is as follows.

LVL 14 in Portal.
Make your way from the starting chamber down the hallway.
Turn left and go up the fancy moving stairs.
At the top, stop,turn around, look back and see that the stairs drop down.
On one side of you (to your right) is a platform with the companion cube, ignore it.
On the opposite side of you from the companion cube (to your left) is a smooth wall. One end joins the back wall, in the rear corner, and at the other end it joins a narrow - 1 block wide wall, in the front corner- next to the pit where the stairs once were.

Basically put a portal onto the back wall in the rear corner way up at the top so that it's facing that small 1 block wide front wall.

Face the pit where the stairs were, step off the edge and put the 2nd portal into the floor where you are falling. You will come out of your first portal at a decent speed and hit that 1 block wide front wall, before dropping to the floor in the front corner.

If you are quick you can re-shoot your 2nd portal to be on the floor in that front corner. A loop results and in no time you have the 300 ft jump. (no time being really about 30-60secs)

To unlock this one, just find any hallway or dead end that is roughly 2x2. Place one portal on the upper wall tile and the other portal on the floor tile adjacent to the opposite wall. Step through the floor portal and the wall portal will throw you against the opposite wall, and you will fall down through the floor portal, starting the process all over again. The achievement should unlock within a minute. (If you are getting "stuck" on the floor, just make sure your floor portal is aligned right up against that opposite wall.)

The 2x2 space I found is in Chamber 16 (Chapter 8), through the doorway and directly to your right after you defeat the second turret.