Short answer: Feed it. ;)
Longer answer: Please do not actively work to "bulk up" your crested gecko. If you are feeding a good quality diet (Like Pangea Complete) and gutloaded insects(I use Crickets and Dubia) 1-2 times a week that is perfect. They will grow and bulk out in time.
Growing steadily is best for the health of your gecko. Every gecko grows at it's own rate and goes through "lanky" stages along the way. There is no need to push them to grow and doing so can actually have detrimental long term affects. Using great food, great husbandry, and gut loaded insects 1-2 times a week is going to keep your gecko healthy and growing steadily. Pushing them to grow faster by offering Higher fat insects or hand feeding to try and make them eat more at a time or feeding almost exclusively insects may make them grow faster...but as is seen in MANY other reptile species "Power feeding" reptiles isn't great...
If your animal looks overly skinny the only real precaution you need is to take in a fecal sample to a vet to look for parasites. Any animal can have parasites brought in by a feeder insect. IF that is the case for your animal simply treating the parasites can make a huge difference and get them on the right track to growing up healthy.
Remember, STEADY growth should be the goal over FAST growth. They WILL grow...just feed them!
Side Note: What is "Gutloading"? Gutloading is when you make sure to feed your insects a good healthy diet of fresh veggies (Carrots, greens, sweet potatoes, etc), Fruits (Strawberries, Mango, etc.), and insect diets like Pangea PIG before feeding those insects off to your geckos. Make sure those foods are offered for at least 24 hours before offering to your animals. By helping your insects be very healthy and have a "gut" that is "loaded" with good foods, you also help your gecko to get the nutrients it needs to be healthy.
