Exactly, a decoder will start testing out the simplest hypotheses, and letter substitution is one of them (especially if newspapers and puzzle pieces frequently publish them). You should aim to making it as simple for yourself as possible and at the same time as complex for them as possible. It's fairly easy to figure out some letters from common and easy words such as 'and' for instance, with 'an' and even 'a' being very common, so keep those sorts of things in mind. From there, whole words can be figured out and so more letters can be filled in. There shouldn't be any hints contained within what you've coded. In some cases people can see if a symbol is more likely to represent a vowel or consonant, and even if they get stuck, they might resort to trial and error. The double letter example for instance. It's way more likely that it would be a consonant. Or else 'ee' or 'oo'. All hints that should be avoided.
You could look up various encryption methods and maybe even mix them, sort of like a double encryption.
I think these sorts of things are fun. Let me know if I sound like I'm getting too carried away.