Don’t skimp. Buy a quality holster that’s contoured correctly, sized properly, and does not require over or under tensioning in order to stay in place. Nerve or vascular damage can occur if the holster isn’t worn properly. If it offers one-size-fits-all or universal adjustment, beware.
Off-the-shelf mass-produced ankle holsters are sized, proportioned, and contoured for average-sized men. If you are not “average”, pay attention to taper and contour for their effect on bone/muscle structure at the ankle and calf.
Counter-balance the weight of the gun with weight on the other side. A spare mag, light, or other weight helps to even out your gait and limit other tendency to “throw” the weighted leg.
Be sure to try all of the different styles and pairs of shoes you wear with your choice of holster, and be willing to change one or the other. I’ve had holsters that worked well with athletic shoes that were intolerable with dress shoes, and vice versa.
Practice, practice, practice deployment.
Style, cut, and construction of pants should also be considered. Finer material may wear quickly. Consider having your tailor reinforce fine dress clothes. To preserve a dress look, increase the rake of your hem with greater drop to the heel. If material and appearance are less an issue, add 2" to your inseam and drive on.
If you carry the really lightweight guns like LCPs, scandiums, and sometimes the airweights, they will mask the failures of lesser holsters. If you want to carry more gun, like a Glock subcompact or an M&P compact, you will need a better holster.
I ankle carry a G27 12-18 hours every day. I use only Alessi ankle holsters to maintain comfort and prevent injury. I’ve been able to carry G19s and G23s on the ankle as well, when the advice above is heeded.