To get a really crisp edge, snip a few small cuts in the fat around each bacon rasher. This will also stop it from curling up too much when cooking.
It's best to buy dry-cure bacon if you can, as less water is released when frying, ensuring really crisp fat (the best bit) and a better flavour.
Place a large frying pan over a medium heat and add a trickle of oil.
Lay the bacon in the pan and cook for about 3-4 minutes on each side or until it's as crispy as you prefer. Remove the bacon and divide between 2 slices of bread. Spread either tomato ketchup or brown sauce on to the remaining 2 slices and place sauce-side down in the pan.
Leave for about a minute to absorb all the juices in the pan, also warming (but not toasting) the bread.
The other 2 pieces will be warmed through by the hot bacon on top, with any bacon fat seeping through the bread. Assemble the sandwiches and eat straight away.