I found remembering the tones for new words really hard at first, too. A while ago, I hit on this method that really helps a lot (for me at least):

Visualise four places that you know. The first should be somewhere high-ish and horizontal - a wide building on top of a hill, for example. Make the second a different place with a big staircase going up. The third should be a low and rounded place like the bottom of a valley. The fourth is a steep downward slope. These 4 places are for the 4 tones.

When you learn a new word, visualise the thing it means (and the parts of the character too) in the corresponding one of the 4 places.

I find this really makes the tones stick - sometimes even better than the basic pronunciation itself!