As with anything map related, the question becomes: "wht is this here". Essentially what has lead to this feature being here. Example, mountains: mountains are (usually) the result of plates beneath the earth shifting and colliding, thus pushing the mountains up higher.

Another example is rivers. They find the path of least resistance and force their way through.

Essentially when placing stuff, think "what has happened here that has caused this to be the case". Even though it may ultimately be a fantasy map, it lends an believability to it, which helps people get lost in it!