Ale, just to make things clear - you CAN'T use 'shall' and 'will' indiscriminately just to vary things. Please don't think that they are the same. There are many, many cases where one or the other of them would simply be wrong. And in many other cases where both words are possible, the meaning is slightly different. It's a very complicated area, and it's frankly one which most students don't need to worry about too much. There are rules, but most of them are outdated and not very useful to learners. The main times that we use 'shall' in everyday modern English is to make offers (Shall I ...?) and to make suggestions (Shall we ....?) Apart from these cases, I wouldn't worry about trying to use the word 'shall'. It isn't very common in speech even in British English, and it's used even less in US English. If you go to http://www.italki.com/question/247178 you'll see the answers which I and another member gave to a similar question a day or two ago. You may find them useful.

I agree with Su.Ki. - "shall" and "will" are not interchangeable. Start off with using "shall" for offers and suggestions, for I and we. "Shall I make you a coffee?" "Shall we eat out tonight?" if you use "will" in these examples, you're basically saying "I don't know - tell me!" You'll learn much more about shall when you reach advanced level, but for now.... offers and suggestions, I and we.

Shall and Will can be used to mean the same thing and yes you can use either. I would use them both if I were writing a letter with a sentence containing both meanings. So that I would not repeat the word shall twice in one sentence. However you shall find that will is mostly used in everyday speech. Hope that helps :)