John, the ride-down problem with tights is common.  The easiest solution is to get an external belt.  Body Basics makes a belt of webbed elastic that is adjustable and fastens with a snap.  There is no website reference for this belt, which retails for less than $3.00.  You would have to try a dancewear retailer.

If the belt I refer to, or one like it, is not available at your local retailer, an alternative product is an athletic uniform belt - the kind, for instance, that baseball players use in their pants.

Actually, any sort of belt will do, not just these specialty items.  But you want something that isn't very wide and won't poke into you or even hurt you.  A belt that may be perfectly appropriate for streetwear might cause harm to someone executing grande battements.

Suspenders will do the trick, and I'm sure that someone else (such as the redoubtable Balletboy) can instruct you on how to make these and fasten them to your tights.  (It's all I can do to sew the straps on my ballet slippers;  suspenders for tights confound me.)

BTW, John - a unitard is about the least flattering garment on the planet for anyone who is "not in the best of shape."  Perhaps, as someone else suggested, you should re-consider tights, especially, black tights.  Remember, black is slimming; so important now, after the excesses of the holidays.