Obviously getting all the gaps is the big one in this achievement. Just to clarify, you need all the gaps in Story AND Classic mode.
I highly recommend leaving your gap hunt until after you have collected all skate ranch pieces and completed all missions. This way the skate ranch has all the pieces required to finish the gaps there. For example I was going spare at myself when the guide was advising me to look for an area with car parts in the skate ranch that I couldn't find for the life of me. It was only when I checked my skate ranch checklist in the menus that I realised I hadn't even completed the mission for the parts. (I have read in one of the other solutions that you don't need all skate ranch parts...which is bizarre as finishing every mission is a requirement and you get all the parts doing that anyway)
Anyway rant over. Your best friend in going for the gaps is Free Skate mode. The reason for this is the story mode terrain becomes altered as you progress and sometimes gaps become unattainable. For example there is a car stand in the casino that for me didnt appear in Story mode but appeared in Free Skate. I'm guessing one of the missions in the story involved using it for the skate ranch so it was moved from the casino. Another advantage to Free Skate is the ability to set a restart point that you can instantly teleport to which can be incredibly handy for gaps involving a long backtrack if you fail them. Options menu=>Game Progress=>Set Restart Point (or something to that degree).
Another thing to bear in mind is cheat codes don't affect achievements in this game
Helps you keep balance when grinding: grindXpert
Moon Gravity: 2them00n
Perfect manuals : 2wheels!
Enter the cheats on the main option menu at the title screen and you can toggle them on or off in the game by going to the options. The perfect manuals and grinding are self explanatory and should always be on. Moon Gravity is fantastic for big jump gaps and the like though you probably want to turn it off as soon as you have finished.
Now for the gap finding. I found the gamefaqs guide very confusing to follow at points then stumbled across these beauties on youtube. I managed to get most levels completed within 30 minutes each thanks to these beauties.(Many thanks to the original poster of those videos the guy made this achievement so much easier
One last final note. If like me you had everything and the achievement hasnt popped try loading up story mode. I finished the gaps in Free Skate and nothing popped. Save your progress and load up Story it should then pop.