While I am not an expert, I have been working on this recently and have a solid validated experience in the past.
I find that the environment you are in is key. If you are in a place that is noisy or distracting or something is going to cause you to wake or move, it will be very frustrating. Until you have really mastered this ability I find that securing a quiet place where you won't be disturbed is crucial. Even something as a cat jumping up on your lap or bed may cause you to return to your body sooner then you wanted.
Start with deep meditation a few times a day so that your body is comfortable with relaxing but not falling asleep. You want to be in the state of haze in-between awake and sleep. Personally I time my trials with the moon phases but I have not found any real evidence that this helps concentration or not.
Have a location you want to reach and know where it is. Start with something close so you won't be out too long. The stronger and better you become the further you will be able to travel.
Again this is just what I have learned in my experience and studies. I hope it helps you improve! Good luck.