Elevation is almost always necessary in order to gain this achievement. You can obtain this elevation by three methods: Getting up on a high cliff or mountain or Volcano (Which provides powerful updrafts), stepping on a Spice Geyser, or, if feeling zany, getting thrown by an Epic Creature.
Speed is useful in obtaining this achievement, as it determines your initial velocity, and thus, how much ground you cover per second. As Sprint increases speed, it is also useful to use prior to the flight (and possibly during the flight).
The presence of a Spice Geyser, either at the start of your journey or during it, is a major boon. It will increase your elevation by an amount proportional to how close you are to the ground: The closer you are to the ground, the higher you'll fly. At a certain elevation, spice geysers are useless as you will simply fly over them.
While it may be possible to complete the achievement by other methods, the easiest method is to get the "Cassoworry" wings (Bottom Row of the Accessories Page), the "The Toadening" feet, then simply sprint, then jump off a cliff. While in the air, hitting space (Glide/Jump) repeatedly will keep you aloft. This, in combination with keeping your creature moving forward with the mouse or arrow keys, will gain the achievement. If this does not work, then you can do one of the following: Find a higher cliff; Aim for lower ground; Only bat when you're looking forward and you can see your shadow; Aim so your flight path crosses over a spice geyser. Note, Spice Geysers become more useful for this purpose as you get lower to the ground.
During flight with wings, you only have a certain number of 'flaps'. Each of these flaps increases upward velocity, and thus elevation. Each of these flaps has the same impact during any point in the flight.
If you want to be plain silly just go up to an Epic creature and run to a cliff. Get the Epic to throw you. It takes time to get him to throw but it is certain to get you the achievement if you run onto a cliff edge when chased by an Epic.
Assuming all other elements are present, getting the achievement without glide wings is possible, but very hard. Sprinting off of a high point, utilizing maximum speed, and then crossing a spice geyser during the flight could make it possible. It is only recommended you try this if you are trying to have some fun.
Due to the fact the program works not by distance of flight but how far you can go without your body touching the ground, you can swim a long distance near the coast and if you are patient enough you can get the award.
Because of the above, landing in the water during the flight will not reset it, as it is not 'touching the ground'. Watch out for the Sea Monster.
If you are fortunate enough to have a volcano on your file, climb to the top of it. There will be a huge updraft of air that pushes you almost to space. If you have even the Antioseptra wings you can get the achievement from an unmoving start.
If all else fails, fly out to a coast approx. 200+ meters away from your nest. fly out to the point where you get eaten by the Sea Monster. When the Sea Monster eats you, make sure you are flying. If you did it right, the game should think that you flew all the way back home and you will get the achievement. It's a little debatable whether this actually works or not, but it is still possible.
Rarely, when your creature is swimming and is eaten by the Sea Monster, you can also get the achievement.
Another method is to use the "Advance to Tribal Stage" option 200+ meters from your nest. While you don't see it, you will instantly be teleported to your nest without touching the ground. (It's when the cutscene starts after you pass through the history page that your achievement pops up!) This can only be done with a creature that is in mid air at the moment you press the button. Same concept as the above method.