What are factors that ruin a time travel story?

I want to avoid the things that make a time travel story bad, because I'm working on one.

Also, what are factors that I should be sure to address/consider during the creation of my story that would make it better?

What do you do when you meet yourself in the past/future? That's always my question when reading time travel stories.

Make sure that your time travel explanation is either really solid or really vague.
Make sure you research any time period you're going to be writing in.

The common hooks are if you go back in time the future will change. So technically you could exist in two worlds at the same time. The old you from the Future and the you from the past. It is funny how in movies this is portrayed because the person when he goes back to the future knows changes have aken place but for him to have been aware of the changes, the him that is still in the past would have to have forgotten everything that happened to that point. Confused yet. See the Denzel Washington Movie Deja Vu. It is a perfect examle of how to do the book. In the real world it is very messy.

You have to decide in advance how to handle the time paradox. You know the one where you kill your grandfather so that you were never born, etc.

From stories that I have read there are several ways to handle this:

1) Alternate time lines - you go back and kill your grandfather, but in your time line he was not killed.

2) The time travel mechanism prevents you from traveling to the "near past" , maybe you can only travel to the future and never come back.

3) You can view events only but not interact.

4) The "God" of time travel or some imaginary "force" prevents you from changing anything in the past. Maybe you get killed or things happen so that you cannot complete your plans.

Trying to work too far outside the conventions or working with too many cliches. Time travel, whether scientific or fantasy based, always plays on the fine line of the absurd but it can never cross. Once is crosses even the fantasy based becomes at best silly.

You need to plan completely in advance the time line and dimensional factors. These will vary between scientific and fantasy based timetravel but here are a few questions to ask yourself:
Do possibilities exist on different dimensional time line? Will causing your mothers death cause you to instantaneously disappear? Will destiny cause changes in the timeline to work itself back on track? Ask yourself questions an keep your theories straight.

Consistency will be very important.

What is found in the 'present' of time travelers must be thought of, and referred to, and gone-back-to-see (or forward-to-see), with consistent views of all 'present' time world happenings, technical happenings, town happenings, family happenings, and any other happenings the travelers see in their present time.

What is found in the 'future' (or 'past') of time travelers must be consistent, as it's found, with each part of the 'future' (or 'past') world in some way reflecting even just a glimpse of the world the time travelers left behind, or went ahead of.
Readers like to know the same world is being seen, just at different times.

The 'laws' you make in reference to the time travel must be consistent.
If you decide that what a time traveler does in the 'future' (or 'past') will affect the past (or future), no matter what it is, that must always be kept in mind by those who 'travel' and know about it.
If something does happen in the 'future' to affect the past, or the other way around, you'll have to decide how and stick to it.

If the 'laws' of time travel you make don't involve 'future' (or 'past') activities affecting the past (or future), your readers may want to know why, so that will have to be clearly explained, which can be done in a good exchange of dialog about the 'travel.'

When you create your time travel story, it's also always good to have the actual mode of time travel well-described, as well as how that mode may 'disappear,' 'reappear' --just what it looks like and how it's used. Consistency is very necessary here as well.

I think you know what I feel is very important in a time travel story now, and others will chime in, so I wish you luck with your story--