Is there a way I can stop slipping my tenses when I write? If you've read any of my work you're know what I mean. When I write I try to see myself as the character (in the past) but then I slip and start writing it as if I'm observing the characters (in the present)-hence mixed tenses. Does anyone know of a way/technique I can use to keep my writing in the correct tense or do I have to resign myself to correcting it later in my editing?
You should probably ask in word mechanics about that. I believe you will find it easier to correct later when you return to edit your work.
I don't think there's a technique other than re-reading what you've written every few paragraphs and correcting for tenses. That's what I do.
I would guess it will helpif you don't write in first person: I did this and that. Instead, always write in 3rd person: She/He did this and that. It's easier to fall into storytelling mode that way (past tenses). You can try forbidding yourself from using now/this as well in descriptions, as this brings the narrative into the immediate. I also find keeping a diary is not a particularly good habit for novel writing, all the "I...I...I". I always jot down ideas in character in 3rd person.
Try reading aloud as you go, you'll find it pretty easy because what you said won't make literal sense to you and you'll most likely stumble.
I have this issue as well. edit... edit... edit... I can't really think of any other way to go about it.