Calorie-density is important. While it's not always about the number of calories, it helps to get it from tasty sources. This tends to be fattier foods that are easy to get down. Coconut milk, almond butter, nuts, cheese (if you can do dairy) and other dense sources are good. I find it much easier to get carbs from things like honey, sweet potatoes, bananas, figs, etc. These things are dense and easy to not fill yourself up with.
Volume is also important, but varies from person to person. Some people fail to gain weight on Paleo because it does require a certain level of food volume. When you're getting starch and protein, you have to deal with some "full belly" feelings. I dislike this, but it is necessary. I've tried to take it in stride and plan my volume eating for a time where I have some time to rest and digest afterward. It also helps to use HCL and optimize digestion to minimize the time you spend in this state.
Nutrient-time is THE key here. Assuming calories are high enough, you can maximize gains by really timing your food. Leangains is great for this, but not everyone can do it. Depending on your activity, really focus your intake after it. Once a week I do max dead lifting with some supplemental squatting and kettle bell training. These are my hardest training days and last anywhere from 75-90 minutes. The 2-5 hours following it, I eat my heart out and chill out in-front of the TV or computer. It's a great feeling and I've seen radical changes in a matter of weeks. It doesn't have to be a daily thing, just a single stimulus per week can be adequate.