I have a 2.5 year old who also has suddenly become nearly impossible to get down for a nap. It wouldn't be so bad, except that he really needs the rest and is not fun to be around if he doesn't nap.
What still sometimes works for us is to make sure he gets a bunch of exercise (or that he is sick! :) ), and then get him to lie in bed to listen to stories. If the timing is right so that he is either not overtired or if it is late enough in the afternoon that he really is tired, he will sometimes fall asleep while being read to.
I have an 8 month old too, which means there is often a baby clambering around and fussing in my 2.5 year old's room when I am trying to get him to nap and it limits the time I can spend on it.
If I really need a break and he won't nap, I tell my son that it is quiet time, shut him in his room, and do what I need to do. This may sound harsh, but we have a lock on his door, so if we put him in there, he stays. (We used to have a baby gate up, but he can open them now.)
Other than that, I have just done some mental adjustment. Rather than feeling doom and desperation descend on me when he won't nap, I'm trying to regard any naps as a bonus and not counting on them. And if my overtired kid won't stop having tantrums, bopping his brother on the head, etc., I'm not above putting him in front of the TV for a little while.
I also have my kid in a small home preschool three mornings a week. It gives him a lot of good stimulation and social interaction, and gives me a break (although sometimes having just one baby to deal with doesn't feel like a complete enough break!). I don't think the "preschool" aspect is important at this age, but it was an easy way to get quality part time daycare.