Baron & Kenny (1986) and Sharma et al. (1981) offer some guidance. Your theoretical knowledge of existing variables and relationships in your current framework along with any related, prior work should also play a role in your selection strategy.
Baron, R. M., & Kenny, D. A. (1986). The moderator–mediator variable distinction in social psychological research: Conceptual, strategic, and statistical considerations. Journal of Personality and Social Psychology, 51(6), 1173.
Sharma, S., Durand, R. M., & Gur-Arie, O. (1981). Identification and analysis of moderator variables. Journal of Marketing Research, 291–300.