Supposing a computer is working its way through every combination of letters and numbers trying to find your password, and it's trying a million different combinations each second. How long will it take to find it?
If your password is made of 4 random lowercase letters, the computer will probably come across it after about 1/4 of a second. If you use both uppercase and lowercase letters and digits, but still only have 4 characters, it will take the computer about 7 seconds. But if you then increase the length to 8 characters, it will take about 3.5 years, and if you increase the length to 16 characters it will take about 755 trillion years.
Increasing the length of your password exponentially increases its strength. So your goal is to create a password which is long, unguessable, but memorable. This page provides some tips.
You only need one password, make it good.
To generate a long password, try taking several of the tips on this page and using them together to make a really long password.
A good way of generating a seemingly random set of letters is to take a phrase that you can remember and take the first letter of each word. E.g. "I like to boil my road cones in ketchup on sundays" gives you ILTBMRCIKOS.
Memorable dates such as friends' or relatives' birthdays can be used, or door numbers of houses you've lived in, old post codes or phone numbers.
TwistPass has no restrictions on the characters you can use in your master password, so throw in some interesting shapes.
If you want to get really obscure then you can add in some odd characters like é or ü, but remember that you will want to be able to type your password on many different devices (phone, laptop), so make sure you can get these charcters on those keyboards! And remember, adding length to your password is the best way to increase its strength.
Passwords based on real words are much weaker, as they're easier to guess. But as you'll be typing your master password a lot, it may be quicker for you to type "correct horse battery staple" than it is to type "mÛ{$3Q", so if you're looking to pad out your password with some extra length, why not sneak in a real word or two.