How to avoid gender bias when writing letters of recommendation?
When I write my letters, I try to consciously avoid the biases that have been reported in the literature. Namely, I try to avoid stressing the effort/emotion/affect of female candidates. For both genders,  I try focus on research capability and competence.

You may find these helpful:


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572075/
http://www.academic.umn.edu/wfc/rec%20letter%20study%202009.pdf
The graph below can be found in this paper: https://www.ncwit.org/sites/default/files/resources/avoidingunintendedgenderbiaslettersrecommendation.pdf 
the original source is: Exploring the Color of Glass: Letters of Recommendation for Female and Male Medical Faculty doi: 10.1177/0957926503014002277 where the caption reads "Semantic realms following possessives. Rank-ordered within gender sets from equal numbers of letters ‘her training’; ‘his research’."