In our tameshiwari sessions, we've been using mostly strapped phonebooks at our dojos.
They're cheap and they're easy to find. Practicing regularly should harden both your hands and knuckles. Remember this is a form of training to break boards... not your hands, nor your fingers, so go at your own pace and don't try to burn the steps. Patience is the key.
Series of pushups on knuckles are also very good to strenghten your thrust
and increase your resistance.
To improve my striking motion, I also do series of weight lifting in a motion similar to starting a lawn mower...
The engine still won't start though... and guys always make jokes on that... "Get a new a spark plug, it might be handy, Hey hey"...
"Deuhhh...Thank's for the tip guys, deuhhh... I really haven't tought about that"...
More experienced people also usually train on sand bag,
(sewed from old coffee bean bag material) but in the end, the sand on our old
one had turned to concrete and the piece of cloth was so ripped apart that we finally came to the conclusion that it wouldn't be a bad idea to change it...
We should get a new sand bag pretty soon. That will depend of course on how fast Shihan Andre can sew us a new one...
Want to try something a bit different in your tameshiwari training?
This a very good exercise to improve your coordination and check
if you can keep a good focus on the target using both hands.
The fact that you're either left handed or right handed makes no difference.
You'll have to practice this on BOTH sides...
Try breaking 2 separate boards positionned in a 90o degree angle simultaneously.
eg. while breaking the board in front of you with your RIGHT fist,
your LEFT fist must break the one sideway.
Then try the same thing the opposite way. Breaking the board in front
of you with your LEFT fist and breaking the one sideway with your RIGHT.
If you have a bit of problem achieving this using single boards, it means that your breaking technique might need a bit of improvement... But hey... that's what training is for... Never give up and go for it... you'll be rewarded in the end.