Any tips on the jump 300 Feet achievement?

I just unlocked Terminal Velocity, now  I need to work on the 300 Feet Jump achievement.

Any hints?

the level with the sentry guns in the cubby holes all around the room. 'nough said

Just find a big open area with enough room to jump 300 feet, build momentum with the terminal velocity technique of matching portals, then when you have enough momentum set the ceiling drop portal onto the ground/wall and jump out of it. That should do it for you easily.

Level 19, giant sentry gun room.  Basically just get it to where you can jump across the slanted piece of wall to the other slanted piece of wall and have and entrance/exit portal on both.  You will go across about 3 times for 300 feet.

On that sentry gun level, if you do it right, you can easily jump a couple thousand feet.  Once you have the portal placed on the angled wall for the big jump, place a portal anywhere on the floor.  Once you've gone through, drop another portal on the ground just before you land.  This should rocket you acroos.  Now, you can even place a portal on the platform you land on which will shoot you mostly straight up(but slightly forward).  Drop a portal on the ground floor again just before you land, and you can keep doing that indefinately as long as you don't miss your portals and don't land on the slanted wall.

For some reason, I've done this many times - and for pretty long - but never recieved any achievement for it.

Hmmmmm, sounnds tricky, but I'll give it a go when I get home.

I had luck on level 14 right in the beginning. You are needing to make a jump to get the box...

Instead, place an exit portal facing the retractable stairway (this must be done BEFORE getting the little crate)... then jump off towards the retractable stairs (which should be retracted), place portal on floor on the way down, this will take you to the exit portal which you will then need to place another entrance portal before you hit the ground... This needs to be done one more time and you got it!

Let me know if this works for you guys like it worked for me (it was nice since there was no having to work any puzzles to get to the place to do it... it's right there in the beginning.

I actually got the 300ft achievement while trying to get the 30,000ft achievement.  I was in the level with the steps that appear and disappear and the block on the pedestal.  I jumped down and came out of the wall and hit it, proceeded to fall into the floor and come out, hit the wall on the other side, and repeat for a while.  I never stopped falling/moving.  Eventually I got the achievement randomly...

For anyone trying to obtain the achievements and having trouble, make sure sv_cheats isn't on (this is switched on automatically in commentary mode), it prevents you from picking up the achievement even if you've completed it's requirement/s.