You can lay them flat, or in trandelenburg if safe to do so and have them turn their head to the side, again if safe to do so. Sometimes you can have them puff out their cheeks like blowing up a balloon and that can help create a backflow in the EJ and make it more visible. Again, all of these techniques are only used if allowed and won't harm the patient. You can also place firm pressure against the clavicle and sometimes that will help as a "tourniquet", EJ's take a lot of practice as a lot of times you won't get a regular flash like you do on other peripheral IV's, it'll be small and you just have to know what you're feeling.
Also something to keep in mind, a lot of ER's don't even allow their RN's to do EJ's and you have to have special training to perform them since there is increased risks associated with the procedure.