The best thing is to divide the application into TCO's(Test
case Outlines)

Example,for any site there can be different TCO's for each
functionality(let's take the login part)

Note:TCO'S are just one liners.eg.
Let me take some of them(let's take the login part)
1>Username and Password are mandatory.

2>User should be able to login with correct username and
password.

3>Username and password should be case sensitive.

4>There should be entry box for each username and password.

5>There should be an option for forgot password.

6>Password should be encrypted.

7)Username and Password should not take white spaces.


Now the next step is to break these test case outlines into
test cases.There can be one or more test cases that can be
written using the tco's you have identified.

For example for tco

2>User should be able to login with correct username and
password.

we can break this tco into different test cases.
a)Login with correct username and password(user should be
able to login)+ve testing.
b)Login with incorrect username and password(user should not
be able to login)-ve testing.
c)Login with incorrect username and correct password(user
should not be able to login)-ve testing.
d)Login with correct username and correct password(user
should not be able to login)-ve testing.

For each tco look for all the +ve and -ve
scenarios.Generally if you have 5 +ve test cases then you
should atleast have 15 -ve test cases.

There may be cases where there is 1:1 relationship between
tco's and test cases as in the case with 1>Username and
Password are mandatory.

This is how i write test cases.May be this is helpful.