I think there is really a big difference between boasting and being positive about yourself, and people who know you like your friends, family and regular co-workers will know you arent a boastful person. So being a bit more positive around people you know really well, should be less challenging than around strangers.

Im no great expert obviously, but I do find a bit of ironic humor can help you say positive words about yourself, while not seeming too boastful. Arrive at the office burst in and say "Nobody panic, I'm here!" with a cheeky smile, everyone knows you are joking, but there is an element of 'yeah, we all know Im joking, but there is some truth behind it too'.

Stay cool mate :)