What I've been doing so far is to actually play other people's games in the jam and provide useful feedback, mention any bugs or even ideas that you think could improve people's games. Personalize every comment that you make! Sometimes I take screenshots of the games I've played too :D
People will appreciate that you've took the time to try out their games and will likely do the same to you too! Asking people to try out your game after you give feedback also boosts the chance of people actually playing your game! But remember, the feedback you leave first should be good. At least 2 or 3 sentences is what I usually do.