Right, I have to jump in here - I'm in the middle of this whole process right now, here's what I've learned!
1) A website is fine, but depending on where you live can get you into all sorts of trouble. If you're asking the public to invest, in the UK you MUST be FSA registered. The process takes 3 years, so partner up with an IFA and ask them to make the approaches for you - do NOT ask for money on your website as this can get you in really serious trouble.
2) Don't even consider sponsors - if your film is worth making, look for INVESTORS. Investors get their money back plus a profit. If you ask someone to sponsor you, they know they get nothing in return, if they invest, they may get their money back.
3) Public bodies do not sponsor feature films. All the schemes everyone says 'ooh have you asked The Film Council/Screen West/whatever for money?' don't give money to feature films. No matter what you think you know, they don't. You may be able to get a grant for a short, but that's because shorts are not expected to be commercially viable. Features are big-boys league - you have to make money. The Film Council may invest in your film (note INVEST!), especially if you need 'completion money', and can prove to them it's good for British Film - specifically that you've taken the risk out of the project (ie it's shot) and that you have distributors interested.
4) To convince anyone to invest, you must have a decent wodge of paperwork for them to read - that means a budget (a realistic budget) broken down to around 40 headings, with sub-breakdowns below that. You don't have to stick to your budget, but you have to have one to be taken seriously. If you can get potential investors into a cinema before midday to see a screener of your (or your DoP/Directors) work, you may sell them the sizzle and romance of cinema as well as an investment.
5) Tax - UK VAT is 17.5%, so if you're VAT registered, you get back all the tax you paid - this helps with a few grand back every now and again whilst you're making the film.
6) Business incentives - UK govt has a scheme EIS where investors get 20% of their investment back in their tax codes (so a £5k investment costs £4k) - even semi-serious investors expect to see this. There may be other tax schemes available to you too (eg FPTC in UK).
7) Be deadly serious - you need to be taken seriously for anyone to invest. People will ask you tricky financial questions, make sure you know the answers. If needs be, partner with a producer who will drive the project (although they may not invest cash they can make it happen)
Not at any cost - some investors want to yank your chain, to them it's a power thing. Get investors who love your project as much as or more than you do yourself!
9) Wheel and deal, make connections, ask everyone you meet about investing in your film. Ask them to ask their friends. Keep asking.
10) Integrity is essential. Always carry out any promises you make. People have long memories, and once you find a few investors, you'll want to go back to them to ask for more money later... Never take anyone's money who can't afford it - no matter how much they're into the romance of it. If someone can't feed their kids one day because they believed in your dream, and it didn't pay off, that's on your head.
11) Be realistic - raising £25,000 is hard work, and £100,000 is four times harder. For every budget line, ask how you can do it for less/free. Buy a camcorder? Hell no, rent one! Pay £500/day? Ask for £400. Paying for a four-day-week? Work them down to a two-and-a-half-day week. Then ask if they'll throw in the legs/dolly etc.
12) Do what you can do yourself, what you can't do yourself, pay someone FAST. Fast workers can cost you less to get a job done than cheaper slow workers.
13) Deferments - ask nicely if people will take some cash now (enough to cover their rent, say, plus food) and their full fee on deferment. Some will say yes, others no. The shorter the favour you ask for (eg a weekend as opposed to a week), the better the chance they'll say yes. Even my lawyer and accountant accepted deferments!
14) Against that, only spend money you already have. Debt is a really bad way to finance anything if you can possible avoid it. Everyone wants to be a part of a success, but if you're in debt, you're on your own.
15) Say yes to things - if someone asks you to help them film a pop concert (for instance), and you're not busy that day, take them up on it - be a 'can-do' person. If you put yourself out for others, they will put themselves out for you - and you can never guess where it will lead, or who you'll meet as a result!
Blimey, I'll rest at 15 for now...