Suggestion from a beginner, which I am, but so excited about learning this amazing program,,,,,,,,
1. I don't use the high quality for renders, thus cutting way back on time. I consider this practise for when we get the final program. But the learning process is just wonderful for me, I sure enjoy it.
2. And for crashes, I learned quickly not to be running all sorts of other programs at the same time as when you are rendering.
Google Earth and Photoshop were my problems, I was impatient.
Just yesterday I had to Force Quit Terragen, my own fault(using Photoshop, Safari, Firefox and others).
3. Use another computer(Ive an older Mac here beside the big Mac), for email, surfing, music and all your other stuff, while the newer one is tied up with TG.
Hope this helps, Ive not much experience with these complex programs, but do find that this does help me anyway. And my classmates too.
Jane

(Good heavens, I almost knocked my computer over, jumped up to see a huge coyote in our yard chasing squirrels and I realized my neighbour's cat was out, chased him away. cat is OK, whew!)