It turns out that there are a number of different ways to speed up this process. The fastest option may or may not be available to a specific character, so let me try and include not only that option but some other alternatives as well. There were already four answers when I posted this one, and I've drawn a lot of ideas from those answers.
Fastest
The fastest way was pointed out by wax eagle. It requires the Two-Fisted Shooter feat. The text for that feat's benefit is included in wax eagle's answer, but note that it has the class Rogue as a prerequisite.
This feat is perfect if you can get it, as it lets you reload the hand crossbow one-handed as a free action. No weapon swapping or free hand needed.
However, if your character isn't a Rogue, doesn't want to multiclass as a Rogue, or is already multiclassing as something else, then you can't take this feat, so you need other options.
Other Feats
There are a number of feats that allow you to swap weapons quickly, which facilitates having a free hand. These include Master-at-Arms and Quick Draw.
Master at Arms
Benefit: You gain a +1 feat bonus to the attack rolls of weapon attacks.
The bonus increases to +2 at 11th level and +3 at 21st level.
Also, you can use a minor action to sheathe a weapon and then draw a weapon.
Master at Arms allows you to switch weapons more quickly.
This would be useful if you didn't keep the hand crossbow out all the time, but instead just pulled it out to shoot. Put your weapon away and get out the hand crossbow as a minor, then shoot and reload with your free hand using a standard action and a free action, then swap your hand crossbow back to the main weapon with another minor.
Alternatively, you could use a minor to put your main hand weapon away, while it's away use a free action to load your hand crossbow with your currently free hand, and then draw your main weapon again as part of the same minor action you put it away with.
So, you can keep both your weapons out, and reload the hand crossbow with a minor.
Quick Draw
Prerequisite: Dex 13
Benefit: You can draw a weapon (or an object stored in a belt pouch, bandolier, or similar container, such as a potion) as part of the same action used to attack with the weapon or use the object.
You also gain a +2 feat bonus to initiative checks.
This is less helpful. It might be useful combined with some of the other options here, but alone all it does is allow you to save a minor to draw a weapon.
Still, you could use this if you weren't concerned with having your main weapon out all the time. You could sheathe the main with a minor, shoot and reload the crossbow, and then just draw the main whenever you need to make another attack with it.
So you can keep the hand crossbow out, have your main sheathed sometimes, and reload the crossbow with a minor.
Class Features
Some classes have class features that allow them to swap weapons more quickly, particularly the Assassin (Executioner).
The Executioner has a feature called Quick Swap:
Benefit: Once during each of your turns, you can take a free action to draw or stow a weapon and then draw another weapon.
This is essentially the same as Master at Arms, except you can only do it once per turn, and it only takes a free action.
So, you could keep your main weapon out, use a free action to swap from your main to your hand crossbow, leaving a free hand open all the time, use a standard to shoot the hand crossbow and a free to reload it, then use two minors to stow the hand crossbow and draw your main.
Alternatively, you could keep both weapons out, use a free action to stow the main, interrupt that free action with another free action to reload the hand crossbow while your hand is free, and then finish the free action by redrawing your main.
This second option is nearly as good as Two Fisted Shooter, in that you can reload without using any of your three primary actions. However, you can only do it once per turn, and it seems to be stressing believability a bit.
Items
There are also items that allow you to draw or stow weapons more quickly, including a number of scabbards, which was pointed out by Frezak.
As an example, the Ruby Scabbard can be found in the Adventurer's Vault:
Ruby Scabbard Level 5
Wondrous Item 1,000 gp
Property: This scabbard resizes to fit any light blade or heavy blade. You can draw a weapon from this scabbard as part of the same action used to make an attack with that weapon.
Power (Encounter): Free Action. Use this power when you attack with the weapon most recently sheathed in the scabbard. Gain a +1 power bonus to the next damage roll you make with that weapon before the end of your next turn.
The weapon must have been sheathed in the scabbard within the past 24 hours to gain this power.
This scabbard and others like it provide the same benefit as the Quick Draw feat without requiring that you spend a feat. Of course, you have to get the item and you have to be using a light or heavy blade as your main weapon.
Summary
So there are quite a few options for speeding this process up, but your best bet is Two Fisted Shooter.
If you're an Executioner, then your class features take care of the issue.
Unfortunately those options may not be available for your particular character, in which case you can take Master at Arms, or if you really don't want to spend a feat and you are using the right weapons you can take advantage of a scabbard.
If anybody knows of some other options that should be included in this answer, just drop a comment and I'll add them.
