If you need to set up a new website or a one-off promotion, you'll need to dream up a domain name that is fit for purpose. Here are a few considerations.

Keep it fairly brief.
The longer the name, the more people will get it wrong, or simply be put off entering it in the first place.

Use relevant search words
If you want to be found through search engines. The domain "cheap-beach-balls.com" for your a shopping site will probably pick up Google searches for "cheap beach balls", which is good. But you needn't worry about this so much if the site is just some sort of landing page for a teaser campaign in offline media.

Make it easy to type and remember.
Hyphens are good here: "cheap-beach-balls.com" can be understood and remembered at a glance, whereas "cheapbeachballs.com" needs a bit more thinking about. Mind you, I'd always register both and redirect the unwanted one, just in case people forget to add the hyphens.

Avoid frequently-misspelled words
because, obviously, people will mis-spell them. Words like "misspelling", for example, which is just as often written as "mis-spelling" or even "mispelling".

Make it a natural phrase
with words in the right order. Don't use "beach-balls-cheap.com" if "cheap-beach-balls.com" isn't available.

Ensure it can't be read in unwanted ways.
I'm sure the promotional pens site Pen Island ("penisland.net") always knew that its domain name would provoke much amusement (and get links!), but it's a risky strategy and not one I'd recommend.

Avoid silly abbreviations.
A "2" or a "4" in the middle of your domain just looks tacky.

Be careful with the alternatives.
If your preferred domain name isn't available, alternatives include going for other extensions such as ".net", adding "the" on the front, or changing to a plural. That's fine (although ensure the site name is changed to match) but do explore who's got the domain name you really wanted. If that's an established site which could even be competition to you, then avoid the "second best" option.

Choose the appropriate "top level domain" extension.
For a worldwide site, ".com" makes sense, but if you're focusing on one area, such as the UK or the EU, go for a ".co.uk" or a ".eu". Prospects in those areas will always click on a web address they know is relevant to their country in preference to one they're not sure about. But register the ".com" too while you're at it.