Soldering heatsink pad on bottom of IC

What I do for prototype boards that I'm soldering by hand is to put a large hole in the pad and feed solder into it with the soldering iron. 2 mm works well.

Solder the other pins first, so that the chip is fixed in position.

The flux in the solder will be sufficient.

Number of holes depends on the size of the pad. One is usually sufficient.

You need a good soldering iron with plenty of heat, I use a Metcal.

To get the most dissipation from the pad it needs to be connected to a decent amount of copper.

This is usually the ground plane, so place vias (no thermal reliefs) from the pad (or surrounding area - see document linked to below) to the plane.
As Leon mentions, putting one large hole in the center of the pad can make it possible to solder it by hand from the other side of the board.

This TI document on power pad goes into some detail about how to do things. Another document here also.

I know this thread is old, but I hope my answer can help others with this question.

I work as a PCB layout engineer, and have designed many circuit boards with exposed bottom die pads. For production-level boards, using a grid of small vias (8-10 mil drill) works best to prevent the solder from wicking through the PCB, but in most cases, adding a large central hole is fine, provided the solder paste stencil has some clearance from this hole. In all cases, multiple vias are far better than a single one to reduce thermal resistance. Remember, the vias and solder are the only connection between the IC and the PCB which is acting as a heatsink. Very little heat by comparison can be dissipated through the leads, especially on ICs that were designed with a bottom die.

In most of my designs, I use a large central hole, but my method of hand soldering is different than the previous answers above, but has proven very effective over the years. The problem I've encountered with feeding solder through the central hole last from the backside of the PCB, is that unless the hole is very large, there is no way to verify that it has in fact wetted to the die, and the percentage of the die which is soldered is similarly impossible to determine. To eliminate this guesswork, I solder it first. Here's how:

1. Apply solder to the thermal pad on the backside of the PCB, filling the central hole.
2. Apply solder to the thermal pad on the component side of PCB, until there is enough to form a very low dome shape on the pad.
3. Place PCB in a clamp in a flat, horizontal orientation. Make sure it's raised off the work surface enough to access the backside of the PCB with the soldering iron.
4. Place the IC on the PCB, as centered as possible.
5. Use the soldering iron to apply heat to the backside of the PCB. As the heat transfers to the component side, it will heat up the solder on the pad, and the die of the IC. When they wet to each other, the IC will naturally center itself (though it may need to be nudged with a pair of tweezers)
6. Pull the iron straight down from the backside of the PCB. Excess solder will pull through the central hole, and the IC should pull flat to the PCB. The remaining pins can now be soldered as usual.