I recently started using Beebem, the BBC emulator and noticed the facility to save the state of memory to disc.
This is a very handy as you can save the game you are playing at stages just before attempting dangerous things, rather than having to start again when you get trounced, you can just go back to the most recent game save.
A stage further from this is to hex edit the save game state and artificially increase your number of lives by locating the correct area where the number of lives are stored and changing it. I saved the game at various stages and then compare the files to find these areas.
However, I have found a neat little bit of software that is free and stand alone. It is called 'Hackit' and you can download it here. I cannot remember where I got it from and it has no instructions with it. However, it is quite handy for finding things such as number of lives and then poking a number into a memory location permanently.
To use the software, simply double click the file and run it. Select the process you are going to look at (the BBC emulator with the game running in it). Then select 'Memory Search'. Enter the number you are looking for (3 for instance if you have 3 lives in the game at that moment). Then click on 'Search'. Go to the game and lose a life. Back in Hackit, change the number to search for to 2, click search.
Keep doing this until you only have one or two memory locations displayed in the results window. Next, click the 'Hackit' tab and then click 'Add'. Enter the memory location number (0x12345 for instance) into the box that appears and check 'freeze to'. Then enter the number you want (3 for 3 lives for instance). Click 'Add'. The number of lives should then be frozen to 3 in your game.
The software is a bit quirky, the additions you make in the hackit window don't appear until you refresh the window for instance. However, it is not bad for a stand alone freebie. It has allowed me to have a lot of fun with games that are too difficult for me to get anywhere with normally.