For me, part of the problem is combat. Since 3e, Dungeons and Dragons is a horrible candidate for play-by-chat, because of its highly tactical, realistic combat rules. This is one reason why I favor Castles and Crusades or other old-school systems -- for their abstract combat systems.
Beyond combat, however, there are several techniques you can use to move things along in your online campaign. Here is one: Planned Travel Expenses.
A GM can waste a great deal of time online roleplaying transactions with innkeepers, tavern wenches, and merchants. Since these transactions might add color to a live campaign, but do little to advance a story, I find it's best to automate them. Here's how:
You can usually divide an adventure into a series of travel events. A typical sequence follows:
- The characters arrive in a town and find lodging, then spend a while recouping and reequipping.
- After being charged with a new mission, the party travels for a day or more to their objective.
- The party spends some time at the site of the adventure.
- The party returns to civilization.
You can reduce this list to two basic categories of travel event: In-Town Stays and Excursions.
At the beginning of each in-town stay, work with the players to determine an average daily living expense, then just charge them in advance for the whole stay. If they stay longer than expected, you can either charge them by the day or work out a new arrangement. You'll want to have a list of common expenses to refer to, like meals, lodging, stables, etc. After the first time you do this, you should only have to make adjustments for different sized cities, or special occasions or living arrangements.
Before any overland excursion or dungeon crawl, do the same as you would for in-town stays, except you may end up with supplies (rations, etc.) in your PCs' backpacks or saddlebags; so take encumbrance into account. In this case, you can't just collect gp if the trip runs long, because vendors aren't nearby.
By eliminating the roleplaying of item purchases, food and lodging arrangements, and the consumption of rations, you can save precious minutes that are better spent either engaged in action or advancing the plot.