The difference is that with Astral travel you're actually into the spirit round. Imagining it doesn't get you into the spirit round.
Tips: Try in the morning. This is due to brain waves; during the night you are going to fall alseeo easier both because of your brain waves and because it's the end of the day. During the morning you'll have more energy so it'll be easier to go into trance. Also, try cleansing the space around you and your chakras. Chakras make the whole proces-especially seleration easier.
Furthermore, you could use incense as it helps to go in trance, or even a kind of crystal or herb. There are crystals who help in AP as well. You should also try AP when you're calm...both spiritualy and physicaly. So make sure you have a peace of mind and no one will disturb you.
Additionally you can use guided meditation or binaural/isochronic tunes. They help achieving a trance state... Lastly I suggest raising up your vibrations, it helps to AP as well.
Good luck, and if you need any further help you could mail me!