I routinely spend money on behalf of my employer, on software and software services. Here are some reactions, which I hope are helpful to folks trying to sell to enterprises.
1) If you want to sell to enterprises, build your product for enterprises.
This means that you must be able to document systems and processes that lead to high reliability and security. Every aspect of the product must be managable by more than one person, in case someone goes on vacation or is fired (prominent enterprise services that fail this: Twitter and YouTube). You must be able to distinguish between "users", who just use the product to do their job, and "administrators", who manage the product for the enterprise. (Don't send "upgrade now" emails to users!) You must support--or be able to support--browser and OS versions that are a few steps behind what consumers are using. If it's client software, you must work on Windows. Etc.
2) Put your answers in email. Don't bother with attachments unless it's to print or sign.
I can't tell you how many emails I get from salesfolks with Word documents or PDFs attached to answer my questions. Just answer my questions, please, and do it in the body of the email so I can easily read and forward. Your product sheet might have the answer, but I don't want to dig for it. The sales process can be a competitive advantage, but only if you actually act like you're a person I'm conversing with, not a brochure-forwarding bot.
3) Don't jerk me around on pricing.
If you publish a low-dollar tier or tiers on your site, let me take one of those levels even if I'm a big enterprise. Here I disagree with Patrick--enterprises ARE cost conscious. Managers are charged with buying what they need. If it MUST be expensive, that might be ok. What's not ok is spending a penny more than necessary. If I see a 50-foozle price on your website of $500/month, and you tell me I need to pay $5,000/month for 50 foozles just because I'm enterprise--see ya. Just because big businesses are rich, it does not make them saps.
4) "No" is rarely permanent.
If I tell a telemarketer at my home not to call me back, and they call me back, I am pissed. But if I tell an enterprise salesperson I'm not interested right now, or I only want the low website-advertised price, it's ok to call me back later to see if I changed my mind. There are a lot of brains in a big enterprise, which means mindsets and needs can actually change pretty fast. That said--respect a timeline you're given. If I say don't bother calling for 6 months, then don't call me next month (5 would probably be fine though). Be attentive but not pushy.
5) Don't try to go around me
If I tell you I'm the person you should be talking to, that still means that I will need to get clearance to spend. Don't try to find the "decision maker" above me. I'm not lying to you--I am the person who is responsible for managing the sales process for the actual decision maker, because the decision maker just wants to make decisions, not read 2,000 word emails from 5 different prospects. I know the decision maker better than you do. Going around me is just going to make me tell them that you are an unreliable loose cannon, and they will believe me, not you.
6) I have assymmetric expectations about communications; unfair, but that's life.
If you send me an email, it might be a week before you hear back. Feel free to politely follow up. I probably feel a little guilty about it, but sales guys are easily pushed down the priority list. But--if I send you an email, I expect an answer very fast. Meeting that expectation can be a competitive advantage. As Patrick says, a perception of responsiveness can beat a perception of size. I have often picked smaller companies over big ones because they answered every email and phone call right away.