Self-promotion is a terrible opportunity. You know your talents, but if you're the one to talk them up, people inherently don't trust your claims. When you need to promote yourself, bring a wingman to give your talk a bit more reliability.
Photo by Gulan Bollsay.
As tips blog 99u points out, it's natural to be distrustful of a claim when the person making it is talking about themselves. While talking about your own talents is sometimes unavoidable (like during a job interview), a wingman can be helpful in mixed company to take up the cause. If someone who knows you can vouch for your talents, that may carry more weight than it would coming from your own mouth:
Any boasts you want to make about your capabilities or achievements are likely to hold more sway if you can find a trusted "wingman" to make the claims on your behalf. This is where the alliances that you form in your workplace or in your industry more widely can be so important: Having someone else be prepared to sing your praises can be a powerful tool in self-promotion.
Of course, this doesn't mean you can never say anything positive about yourself, but it can be better to work in teams. It's also a good idea to talk up those around you as well. As you lift the profile of those you know, they're in a better position to lift you up as well. If you have a group of people you work with frequently or know well, what's good for one of you is good for everyone.