Most people are bad at this but Brits are the worst. British people love the self-effacing, self-deprecating understatement typified by Hugh Grant in Four Weddings or Notting Hill. Anything braggy and people could think you are brash and pushy and probably foreign - like the Americans. Or the South Africans. Or even worse, arrogant. Not good.
So how can you achieve the fine balance between not underselling yourself and not overselling yourself?
Selling yourself is itself a scary thought. I don't mean selling a product or a service - we do that all the time either for work or to family and friends when we recommend things - buy this, go see that, get the big box- and so on.
I mean when you are the actual product and you can't hide behind a business card with a fancy title - like within the company where you work. When you put yourself out in the open for all to see it's different.
But selling yourself is vital in getting a job, doing a job and keeping a job. Whether you are pitching for business, giving presentations or surviving office politics - being likeable and competent counts.
Know the product - you
Any good salesperson will tell you that you need to know everything about a product before you can consider selling it. Selling yourself is no different. You need to know all about you, what you are good and bad at and what people like or dislike about you. Do a real and unbiased audit of yourself - then act on the results. Get friends and family to take part - do it online via SurveyMonkey or similar and explain to your friends and family why you are doing this weird thing. You may not like all the answers, but it will be incredibly useful and most people will be honest because they care. Unless you are a jerk.
Don't be all about you
Be a listener and be generous with your time. Even if your job description is to be the most aggressive Teflon-elbowed Alpha type, you will have colleagues and bosses who will only tolerate you as long as your last month was exceptional. Get some emotional credit in the bank for when your business performance sucks.
Tell good stories
No one like or believes a bragger who tells boastful war stories. Weave your achievements into indirect stories that are not patently self-serving - so your amazingness is self evident from the story, not from your chest-banging. Don't tell them all the time. Likeability first - subtle tales of your wisdom and wonderfulness much later and accompanied by humour or modesty or both.
Get the basics right
Keep your hair and your hygiene right. Smile and be (properly) interested in other people. Shake hands properly and dress appropriately. Do things for people, send thank you notes, greet people and ask how they are - and mean it, smile and show those (clean) teeth. Don't slag people off and don't indulge in gossip or office politics.
Be a human
Take your job seriously, but not yourself. Be about more than work. Be a human, have other interests and share about them - this will stop you being dull and boring. Do things for no personal profit - weirdly you will profit the most from those activities.