My former boss who is a manager in the company regularly asks me to do work for him. I have that under control as my current boss backs me up and I can take on the assignments I find interesting and valuable and turn down the ones that aren't. However; mixed in with these requests are assignments to speak or present at industry conferences or trade shows. My former boss over commits himself and is regularly passing these talks/presentations onto his staff, and he continues to do that to me. He always presents it as a 'development opportunity' or 'way to get more exposure'. In reality, he commits and then tries to get out of it himself. I know what I want to do; which is only take on the assignments that are interesting or of value.
My question; how do I turn down these requests, politely, because I need to have a good relationship with this former boss, as he is characterising them as benefits to my career?