English has remarkably little entropy. A letter in a sentence like this one has an average of 1.3 bits of entropy. To improve the secrecy and randomness of passwords, I recommend substitutions. Take, for example, 'A bird in the hand is worth two in the bush.' This can be mixed into, '@B1rdInTh3H4nd1$W0rth2InDaBush!'. Perhaps not as easy to remember, but after you've made a few passwords like this, it becomes second nature. In my experience, it also becomes easier to mentally 'chunk' passwords, so something like 'B3hold0bli1v1on1$@Hand' is fairly simple to recall.
Of course, this brings us to a rather interesting junction. The second sentence has more entropy, making it more resistant to cryptographic analysis. The first sentence has more letters, making it harder to brute force. (Though it has no numbers or symbols, but lets set that aside.) Which is better? Depends on your purpose, I think.