The only safe way to boast is by constructive actions.
It has been said that it’s not boasting if you can really do it. This may be true, but a far more persuasive argument is made when you do it first and talk about it later. Besides, good things that are said about you always carry more weight when they are said by someone other than yourself. When you find yourself tempted to wax eloquent about your achievements, force yourself to pause for a moment, take a deep breath, and ask someone else about their achievements.