As soon as you are able, start upgrading your habitats.
Spend weekends working on timed events, regularly playing so that you have a better chance of earning the great rewards offered.
You can give gems to your friends, and they can return the favor, so make sure that you are regularly befriending people and spreading the wealth.
The type of dragon you send out on an adventure will determine the type of rewards received, so try to send a high-level dragon of the proper element out and watch it return with bonus resources.