When you sell something to yourself at "cost" price you are stealing profit from your own business - until you realise this you will never be successful.
If your systems and processes are not consistent with record keeping and stock security you will never be successful.
If you are too mean to either buy a register with a manual or buy a manual for the register you already have you will never be successful.
You need to be able to account for everything moving through the store and calculate the total income you expect from all the stock at any particular time - then comapre with the income made some time later - the difference will give you the scale of the problem you have to solve.
Get your spreadsheet working and do a 100% stock check - calculate the total receipts from this stock if it were all sold. Do the same one month later and calculate the difference. Compare this with the takings over the period - the difference is the shrinkage. At the moment you are a cause of part of this shrinkage.
Good luck...
Peccavi
January 2010