Test your website in all browsers.
This is a pretty obvious point to have in your checklist when testing your new website.
Although, it can be easily overlooked and I'll be the first one to admit it!
I've had my website launched for over 2 months and sales have been slow, even though I have (what I believe to be) a great product.
Within those two months, I didn't realise that my 'buy now' button, 'subscribe' button and my contact form, don't work in Internet Explorer!
I can't believe how many sales I could have missed out on.
So, cross browser compatability is very important.
Learn from my mistakes!