Those numbers on the datasheet are the manufacturer test conditions. The methods given above for finding the needed base current using the Hfe are still sound.
One thing that free_electron said, that people should heed, is look at the curves.
...the transistor is non-linear and you really need to look in the curves
This is the most important thing to take away from this thread. You can't just use the Hfe value from their characteristic tables. I've seen too many websites do that. For example, the BC847 linked has Hfe of between 90 and 170 for just 2uA of collector current, and between 180-520 typical for 2mA collector current. I've seen people use these numbers for any current, and it doesn't work that way, "the transistor is non-linear"
So, for 100mA collector current, when you look at the curves, you see at room temperature the Hfe is just 80.
Then 100mA collector current with Hfe=80 is 100/80 = 1.25mA base current. minimum. But manufacturing processes cause slight differences, and you should never use this minimum current. so, the rule of thumb was devised: multiply the base current you get using the Hfe by 2x up to 5x , as Psi said. In this case choose 4. 4 x 1.25mA = 5mA. And 5mA is exactly the test current used in the manufacturers tables on page 6. So in their test, they used the rule of thumb too. And yes, it will guarantee saturation at 100mA collector current.
free_electron also mentions Vbe(sat) This is important, especially at lower voltages like 3.3, 2.7, 1.8 I/O devices. Lots of websites online ignore Vbe(sat). At higher voltages you can probably ignore it because you've already factored in enough extra current by bumping the base current by 2-5 times more than calculated.
But at these lower voltages you'd be best not to ignore Vbe(sat), because now it's a larger percentage of the available base voltages. In this case, at 100mA collector current, and 5mA base currnet, the Vbe(sat) is 900mV. If you are using a 1.8V device, it's more than 50% of your voltage budget, so you can't ignore it anymore.
(do they make a transistor with a Schottky B-E junction? I've never used one, but it would be useful as voltages get lower. Maybe we go back to germanium now)