The sandbag is a primal
piece of equipment - just a big bag of sand. I use
mine all the time - you don't need to be too fancy
with it but if you're looking for a GOOD sandbag,
with handles and an inner bag and something that won't
leak all over the place. You can actually buy those
from Staley Training, too!

Click
here for more info on those
I HIGHLY recommend then.
So anyway, this is how
I do a chin-up with a sandbag. If you can reach the
bar from a standing position, you won't need this...you
can just get the sandbag into position then start
pullling.
I'm just slightly too
short to do it with my bar, so this is my setup. Set
a bench in front of the bar and lay the sandbag on
it, perpendicular, like in the picture.

Stand on the bench with
one foot on either side of the bag then reach down
and grab the ends of the bag.

Lift it up then squeeze
it between your knees then cross your left foot over
your right ankle. This locks the bag in place - you
won't be able to just pinch the bag without the cross-over
- trust me on this, especially when you're using a
70 lb bag.

Once you're locked in,
grab the bar.

Lower yourself down,
keeping that bag squeeze tight. It won't matter if
it shifts a bit - it's sand. It'll shift.

Then just pull up.

When you're done, just
uncross and drop the bag. Easy. You can even do a
literal drop set and keep doing a few unweighted reps,
too.