1. Learn jass that makes it loads easier
2. Understand maths and values well
3. Understand how values can represent objects like heroes.
4. Use your head and plan a system before even atempting to start.
5. Structure your S/L sytem's triggering carefully making sure it is as complex and ununderstandale and bug free as possiable.
Incase you do not have a clue how to start to think of how to make one this is a brief discription.
Firstly get your primary vales (hero, lvl, money ect)and convert them into a string form while reducing length.
Secondly Write a check sum by using a mathimatical formular that fetures player name so the codes are name locked.
Finaly run an encrypter that mixes up the values and changes them so that it is hard to interprite but of couse your load generator can.
Or you could try dling and using one of the save/load systems in the spell section ofcourse :?
But I recomend writing your own since all the ones on this site are easily hacked using JASS EXTRACTORS.