In one of the stories I'm writing at the moment, when the narrator is in the scene, the story is in present tense (which is painful, but so far I like the result). The rest is in the much easier to deal with, and generally superior, narrative past tense.
At one point I had given up on present tense and started writing in past tense, however I have since realized that I actually had a good reason to write those scenes in present tense. So I go back and rewrite them paragraph by paragraph. In some spots I just wipe the whole scene and start over, but for the most part I found that wasn't necessary.
I'm now going through and editing those scenes and finding that I keep slipping into narrative past. The entire purpose of rewriting was to get rid of past tense, yet somehow I am rewriting parts of paragraphs in past tense anyway! This is very frustrating. I don't mind it when it occurs at the very beginning of a scene. To me that's just catching up to the present, however when I'm going back and forth within a scene, that's inexcusable unless a character is explicitly reflecting on past events, and even then that, I feel, should be done with care.
So the question is, how do I avoid this slippage? Other than practice, which is in the works.
P.S. A "problem," if you want to call it that, is that consistent is required. For most of the scenes, either both tenses can work out just as well, or present tense is giving me better results. There are some "down time" scenes, I suppose you could call them. For example what I'm trying to fix right now": sitting around a camp fire. These would, as Mark Baker suggested, work out better in past tense. However I can't have it both ways, or so I believe (I may be wrong).
P.P.S. This is not a question of grammar. I can go back and fix incorrectly conjugated verbs as I have done in this very question (as Cloud pointed out). The issue is structural. The narrative, mid paragraph, takes on the structure of narrative past tense. I have a similar problem when I'm writing in past tense if get excited. In such cases I slip into present tense, but these are easy to fix since, as Baker is quick to point out, past tense is like a Swiss Army Knife. Its usually the tool for the job, and when it's not, it's pretty close.
P.P.P.S. Okay, so lets see if I can make a simple example. If it's too simple it won't work.
John walks his dog and
John walked his dog are both perfectly reasonable sentences, but they are also quite simple. Compare
while walking his dog, John stops to smell the roses against
As John was walking his dog, he stopped to smell the roses. There is more to these sentences, and not only do the verbs need to be conjugated differently, but also the conjunctions "while" and "as".
Now, the past tense
The following day, while walking his dog, John stopped to smell the roses is fine, however if we want to convert that directly to present tense we're going to have do some tinkering with the structure. The inclusion of "the following day", as far as I can tell, doesn't really have an present tense analog since there is only "now". A character can reflect on past events, but the story itself is always in the present.
This isn't a great example, and that last sentence does have some issues, but I think this illustrates my point.
PPPP.S. As Cloudchaser pointed out, that is not the greatest example. Or even an accurate one. In a lot of ways this is an extension of a previous question of mine. Without copying and pasting collections of paragraphs I don't think I can illustrate exactly how this is going wrong. My question is the same as before -- how to avoid switching from one tense/structure to another aside from the age old solution of practice.