Its always good in sales to respond promptly to people and keep on top of a million different things at once. It is worth noting that many of your daily tasks arrive through the Inbox of your computer, or on your Blackberry. So when I saw Lifehacker posting "Treat Your Inbox As a To-Do List" it hit close to my heart. Basically if you don't empty your Inbox by actioning every item (actioning could be deletion or filing), you make it harder for yourself to get through all of the responses your customers are waiting for.
I don't actually mean that your Inbox should always be empty, or that you need to respond to emails immediately. That depends much on your activity level and whether you are expecting important messages or not. What you do need to do is make sure you get all the way through it every day or perhaps 2-3 times a day. Just sit there and one-by-one action every piece, respond to each message or schedule time to do it, if it is not something you can do right away. In fact the following book describes this methodology in greater detail, taking it further than merely the inbox.
The Personal Efficiency Program: How to Stop Feeling Overwhelmed and Win Back Control of Your Work!
This is something broader than Sales, but the reason it is so important in Sales is that every question your customer asks, every meeting that needs to be scheduled, every demo request usually involves several people, and leaving an email for days delays everyone involved. From the Sales side, you often should double action every email, as in responding by email, but also calling to check on the response's adequateness, and checking if there wasn't some hidden meaning in the question. Often you will want to forward a question to a colleague to check their ideas on it. However as you do these processes you should move the email out of the inbox and into a folder for the next step of your process to reflect your individual workflow.
Some people tell me that they have a bunch of emails of low importance, but that they want to keep hold of. This is fine and perfectly ordinary. You may not have time at 9am to actually respond or even read all of these messages. Have a folder in your mailbox to put these messages into, and spend some time later in your day or week to read and process these. However the general rule is that if you handle each mail only once, you save a lot of the overall time.
A final point is that in today's world you don't necessarily have to be in front of your computer or desk to do this email actioning. I manage some days on the train to file or respond to many emails on the train to the office which gets me that much further ahead during the day.