Computer music puts out a lot of decent free (with purchase of the magazine) samples as well as the Reddit edmproduction monthly competition packs.

I would like to echo the idea of plotting out the arrangement as fast as you can. While not absolute, in most instances where I fiddle around with a loop for more than a few minutes, the song never goes anywhere.

I think listening to the same few bars over and over again can cause writing fatigue. Don't get too attached to any particular section of the song until the general arrangement is complete.

I also find practicing covers or layouts of other songs helps a lot. You are bound to learn techniques you may have not thought about until you physically try and recreate something.