Tip - drink all your liquids AFTER you've eaten your meals. Otherwise it takes up valuable space on your stomach. Same goes with vegetables - if possible, eat them after everything else.
Try and drink some calories - make your own weight gainer for example. I've done this and it's extremely easy to get the calories.
Of course, vigorous training such as sprints or high intensity 'cardio' will also increase ur appetite.. you'll be begging for a burger afterwards