Burnout Paradise

You can change what all you have to look for in each part of town: http://www.senocular.com/burnout/

OK what you've gotta realise about paradise city is that down every shortcut is usually 2 smash gates (one at either end of the shortcut). So basically you have to go down all shortcuts and take out the gates at both the entrance and the exit to the shortcut.
Here's a tip that I have given to many people when helping them with their smashes...
Once you've gone through the yellow smash gate make sure you turn around and drive through all of the gate so that it falls down and you can no longer see any of the gate standing as when you go down their again there won't be a gate. Doing this can feel a pain and isn't compulsory for the achievement, you just have to go through each gate once. The reason why i'm giving this hint is that in the long run it will make the remaining smash gates a hell of a lot easier to find as when you get to about 390/400 you can focus on the gates you have left. They will still be hard to find but not hard to notice as they are bright and now after an update flash too. You will have to make sure you drive down every road just in case you have missed a small gate.
Also there are smash gates on the top of a lot of parking garages if that helps.

Oh, and you can check all of the things youve discovered (Auto repairs Gas Stations, Jumps, Bilboards, Smashes) by going to the start menu and then look for 'Discovery'

Hope this helps

I found a great walkthrough for all the smashes. It has directions to and from each gate and pictures of each one.

http://burnout.wikia.com/wiki/Smash_Gate

I take no credit for the guide, hope it helps anyone still struggling with finding these god forsaken gates

- Just adding this to H0B0 Santiago's KurtRuzzell's solution because some people do not know -

There is a glitched smash gate that keeps some players from getting all smashes in Downtown Paradise. Near the place where the bridge to Big Surf Island appears is a car park (Street: Angus Wharf). On its top level there is a Super Jump ramp - which you can use to jump across Angus Wharf to get onto the other car park - with a smash on it. When you go through to get the Super Jump your game might not register the smashed gate and you won't be able to smash it down again because it's gone. It might also be possible that it's gone because players took it down when you were playing online (according to some people reporting the bug to the developers).

If you happen to own the Ultimate Box, you're lucky. You can delete the Update (1.9) from your hard drive, start the game without updating, enter Paradise City, quit back to the main menu, start a Party game (which respawns all the gates and billboards) and after that go back to Paradise City again and go for the glitched gate (just be sure to hit the gate without jumping the ramp).
I don't know if there is any possibility to respawn gates if you only own the "normal" version of the game because you can't play Party mode without getting the update (I think).

Even if you are missing another gate (which should be a very rare case because the car park was the most reported one on Criterion's bug list), you should now be able to go for it again without restarting the whole thing! You just need to know which one or in which area/district it is missing to greatly decrease the work to be done.

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reason for last edit: changed gamertag in the first line and added last paragraph

This image has the whole map with all collectibles. (smash, billboard, events, parking garage, junk yard, repair shop, gas station, paint shop, and super jumps

EVERYTHING!