Do you mean your reputation overall, or your relationship with a specific faction, as these are two different things.

If it is relationship, I would be interested in getting some advice on this too, as I didn't pay it much attention at the beginning of my campaign, and now I have poor relationships with all my neighbours and I can't seem to improve them.

Also in my game many of the factions who have broken alliances previously with any faction are shown as having a poor reputation. So I suppose improving relations with these factions isn't going to help that much.