I have almost finished developing an iPhone app which I feel has a lot of potential.
The problem is I have no money, and I am pretty clueless about marketing. So I feel if I tried to release it by myself, no one would hear about the app and I wouldn't be able to get many sales.
Most funding options I have seen for startups are for large amounts of money. I suspect this app will only need at most a few thousand pounds for its marketing.
What options are available to get some funding for marketing the app? Or are there marketing firms out there that would be willing to market on a commission basis?
The strongest marketing technique available is to get each of your users to recruit/spread the word to new users. This enabled hotmail, skype etc. to get a huge userbase without spending a single dollar on marketing (I know this for a fact with regards to hotmail). This mechanism is possible for a range of software or physical products. I once read a whole book about how to do this, there are so many ways.
Keep in mind that there is a difference between Marketing and Promotion. The methods mentioned by User3997 are get advertising/promotion methods which is part of marketing.
Before you get into any of that, take a step back and learn about your market. Think about who would buy this, why they would, who are your competitors, how are they advertising, what makes you different?
From there you will have a better idea of what it is going to take to promote your product. It is very dangerous, from a time and money standpoint, to not consider your market first. So either take some time to learn about marketing or higher a marketing firm, it depends how serious you are about needing to make money with this product.
Those are my thoughts.