Two, well thought through sentences, is all it takes. The first should show the value of your offering and the second puts that value into context, positioning it so the customer understands what's in it for them and how that compares with the other possibilities open to them.
Sounds simple enough, but let me break that down further.
First sentence - Why what you've got to sell matters
a) Who is it for?
b) Why do they need it?
c) What's your product service called
d) Statement of benefit
Second sentence - Confirmation that your customer is making the right choice
e) Position your product against your main competition
f) Show what differentiates your product (what makes you unique/what makes you different?)
g) State how this is money well spent/prove that benefits can be delivered
E.g. For a)start ups who need b)a marketing resource to increase their visibility in the market, Marketing123 is an c)online, 24 hrs a day network of d)marketing professionals and self help resources who can answer your questions and provide fast solutions anytime, anywhere.
e)Marketing consultancies generally involve long term contracts and take time to show return, but Marketing123 f)lets you pay by the hour as and when you need us and gives a 100% money back guarantee if you're not completely satisfied. g)We've already helped over 700 start ups this year and 99% of our customers would recommend us to their friends as a one stop shop for all your marketing needs.
Value propositions are a great way of ensuring you and your employees are all clear on who your company is and what you do for your customers. I would urge you to write one for each of your products/services.
One final tip. Once you've crafted your value proposition apply the 'so what?' test. Ask yourself whether what you've written describes real customer benefits and clearly states how you help customers solve their main problem. If not, keep tweaking until it does.