Originally Posted by Happy
The best approach is just to buy the subway tickets as needed, or use a cash card which starts at 1000 Yen. Again, it cannot be interchangeable between different companies.
The "cash card" (I presume you mean Passnet?) most definitely is interchangeable between all private lines in Tokyo and it's extremely convenient, it'll figure out the fares for you and even give the appropriate transfer discounts where applicable. The only lines where you can't use it are JR railways and the Tokyo Monorail to HND (which is owned by JR).