I don't know a whole lot of hand/finger strengthening exercises, but it's definitely true that exercising those muscles will make playing easier. I've had to strengthen my hands because of a joint condition I have that makes my fingers prone to hyper-extension if I'm not careful. It might not sound so bad, but I remember bending the middle digit in my pinky backwards while playing one day, it was very painful. I bought one those Vari-grip handmasters a few years ago. It allows you to exercise individual fingers as well as your whole hand. I noticed a marked increase in my hand stamina and speed, and I hardly ever get cramps from playing like I used to. I can still bend my fingers in weird directions, but it doesn't happen as often.
It's also important to add stretching to your hand exercise regiment (not sure if the exercises you read about mention it).
There is an exercise you can do that doesn't require you to buy anything; it will both stretch and strengthen your hand:
Start on your E string and fret the first fret (F) with your index finger. Count to ten slowly ( 1 mississippi, 2 Mississippi, etc...). Fret the next fret while keeping your first fret fretted. Count to 10. Repeat until the first four frets are covered (F-Ab)
Then slide your hand up and repeat, starting with your index finger at the second fret (F#) and ending at the fifth fret. Repeat this entire process until your pinky finger is fretting the twelfth fret. Once your reach the twelfth fret, move to the A string and start over. Do this across all the strings of your bass. It's a good way to stretch and strengthen your hand, although its a little slow and boring.
I do this pretty often, although not every time I practice. Hope you can add it to your regimen!