Further as in distance: Here is a way to remember. When you go on a long trip you may eventually hit a piece of road that has a bunch of switch backs or U turns. Thus I may remember going fUrther with a U.

Farther as in moving fArther as in from point A and Advancing to part B. The A in Advance  partners with the A of fAther.

If you have an alternative way to remember the spelling on this word, please share it in the comment section this blog. Thanks.

Addition:

Further can also mean additional. Envision the u in the word as a cup. You use a cup to add further flour when making bread.