It depends what your product is... you could launch a PR campaign or you can hold an event...

Again, it depends if your product is tangible or not or if your are selling a service

Depends entirely upon your budget.

The question could be better answered if a little more qualification were added e.g. how do you successfully launch XXX product to a potential market of YYY size with a budget of ZZZ?

In this context, "best practice" can be boiled down to: have deep pockets.

I think the answers above are both great. Additional context is always helpful. However, I'd be happy to make some recommendations.

If you are trying to a launch a digital product, service or potentially even in the case of a physical product, leverage the power of intrigue. Make your product or service seem exclusive.

What do I mean?

If you've spent anytime in startup circles, you've probably heard of things like Beta List (for getting beta testers) and Product Hunt (new shiny products and services sort of like Y Combinator Hacker News meets Reddit). If you can get featured on those platforms you are likely to get traffic.

Also reach out locally to your network and or bloggers in your industry. I feature local startups on my blog Startup Chucktown when they provide me information (I even do it freely to help spur on the local startup scene)

Also, cost. It is a factor. However, in this day and age with technology you can do most things on a small budget and still pull off something great.

You could try using a hashtag on Twitter and then using Periscope to live stream the launch party. You could shoot videos of your product or service with nothing more than your phone and post it on Youtube and then blast it out across your social media accounts.

You could throw a party at a local bar or pub and buy the first round for the first 50 or 100 people. There are lots of ways to make an interesting launch while keep costs low.