Wearing a t-shirt is to advocacy as having a bumper sticker is to political activism. It's about the least you can do, and isn't likely to change anyone's mind.
On the other end of the spectrum, attending government meetings where the issues you care about will be discussed is the most effective way to bring about change and to make your councils and commissioners actually consider the issues. When you do this, be thoughtful, don't repeat yourself, and present the issues as you see them. Be firm and polite, especially when the opposition present, if any, has the floor. Stay on topic; don't try to beat the drum for every cycling issue. Try to address every point raised by the opposition if they had the floor before you. If they raise an irrelevant point X, when it's your turn to speak, "The opposition's point X is irrelevant because..." It's most effective to attend this meetings en masse; it's hard to sit up on the dais and tell a passionate, organized, and thoughtful group of constituents simply "no", especially when they're right.
...Which brings me to volunteering with local cycling advocacy group. By being a member of such a group, you can keep track of local context for cycling issues (as opposed to BF, which brings attention to many important issues, but not in a much wider context). The organization can discuss the issue and formulate a coherent position, then send a coalition of members to the government meetings. (Or at least compose a letter or email to get into the record and put in front of the board in quesion stating the groups position and reasoning.) This is not much fun (compared to "Let's ride bikes!"), and burnout for such issue-oriented groups is a problem. More volunteers means less burnout, and more volunteers means when someone inevitably succumbs to burnout, there's likely to be someone else who can step in and take up the slack.
Finally, riding lawfully is among the best things you can do in support of your activities with local government. Anti-cyclists will tell government official they should rule against cyclists on cycling issues because cyclists flout traffic laws, hinder traffic, and are a danger to themselves, motorists, and pedestrians. It's a strawman, but it's likely to come up. Riding lawfully means you can rebut those claims with sincerity.
Lincoln, where I live, uses citizen advisory groups to provide guidance to the city council and the mayor's office on specific issues. (There's such a group for bicycling issues.) If your municipality has such groups, put your name forward as being interested in joining.