To underline Joy's point, the modus operandi
has to be to stop being so passionate about your company just for a second and think about the investor first: How can you either disrupt the flow of his/her day (in terms of sheer impact of message), or how can you charm them into some form of 1-2-1 dialogue through something a little more empathetic (probably about them?)
I was at an event recently where a sophisticated angel investor summed up the process neatly by stating that he spends the whole time with proposals getting to reasons NOT to invest in projects as efficiently as possible! Your e-mail header is likely to be the first gateway... ;o)
Finally, another hard-learned tip: You are not selling your product (even if it IS the next Facebook etc. etc.), you're selling an investment. These guys are looking for the the 1 in 10 that will actually deliver a return, letalone transform into the next Unicorn. The smart ones are real pragmatists.