"Best practices for increasing" or "Best practices in increasing"?

This would be an opportunity to share some of our clients' best practices for increasing ridership.

Should I use for or in? Does the choice of preposition influence the meaning?

Both best practices for X and best practices in X are OK; which one will be more apt depends on what X is. If X is the goal that the practices are trying to achieve, then for is called for. On the hand, if X is some activity in which these practices are to be followed, then in may be apt.

In the OP's example, increasing ridership is presumably the goal that the practices are trying to achieve, so one should indeed say 'best practices for increasing ridership'.

It is not essential here what the grammatical form of X is. One would still use for if one were to say 'best practices for the maximization of the number of riders', because the maximization, now expressed by a noun, is the goal of the practices.

If one were to say 'best practices in increasing ridership', that would mean that these are the practices to follow in the course of increasing ridership, but it would not be clear that the increase itself is the goal (while increasing ridership, one could be pursuing some further economic, social, political, etc. goal).

If "best practice(s)" is followed by "for" or "in" depends on what the rest of the sentence is.

If it is followed by a noun, then "in" is used:

the proprietors are keen to ensure best practice in food preparation, storage, and serving

Example taken from Oxford Dictionary

If it is followed by a present participle, like in your example, then "for" is used:

A husbandry guide would be produced too with the aim of setting out best practice for keeping and breeding the creatures in captivity.

Example taken from Collins Dictionary