New versions of
Mist) will do a fast sync the first time they startup. However, if you quit
Ethereum-Wallet before it is done syncing it will switch to slow-sync from then on because fast sync can only be done on first launch.
Fast sync is much faster than normal sync, it should only take a couple hours on a modern internet connection and computer. Because of this, it can often be worthwhile to restart your sync from the beginning even if you are a good chunk of the way through syncing if you are in normal mode.
Luckily, it is possible to "start over" and sync from the beginning.
chaindata folder. Instructions for this can be found here: https://ethereum.stackexchange.com/a/6551/704
Ethereum-Wallet and any other Ethereum software you may have running on your computer (Ethereum related websites can remain open).
Rename, move or delete the
chaindata folder. Personally, I delete it when I am following this process as its contents are completely recoverable. However, if you are uncertain or uncomfortable with this you can simply move the
chaindata folder somewhere else or rename it. If this process doesn't work you can always move/rename it back and pick up where you left off before attempting to follow these instructions.
Ethereum-Wallet. It is critical that you launch
Ethereum-Wallet first after doing this, not something else like
Ethereum-Wallet will start its own copy of
geth behind the scenes for you in fast mode. It is also possible to launch
geth in fast mode with
geth --fast, though for beginners I recommend using the UI rather than the command line.
Wait for it to sync. As mentioned earlier, this process should only take a couple hours on a modern connection and computer. You should see the block numbers going up much faster than before, assuming you were in normal mode previously.
Ethereum-Wallet! Leave it open and running (you can minimize/hide it) until the initial sync is complete. If you quit and re-launch it you'll restart in normal (not fast) mode and you will have to follow these instructions again if you want to get into fast mode again.