To being with, first and foremost use Common Sense. For the person walking in... When you are out IC, you are in character. Remember that at all times. You act as how your character would act, you're driven by your characters motivations, you see the world through your characters eyes. Now, first off, before you go out its a good idea to take into consideration things like what your character's mood is that day, what your character is wearing, what your character may be planning to do that day if anything. All this really should be somewhere in your RP-bound mind as you're about to leave, and should be kept in mind throughout your IC ventures... rather than just suddenly having to make something up and then being inconsistent.

Now when you're actually out wandering, before you walk in somewhere you have to think, "Would my character ICly have a reason to be in this place at this particular time of day? If so, what would my character be doing? And what would my effect be on my surroundings?" If you're a 8 foot tall neon pink hideously deformed mutant, would you really be out in a shopping district in broad daylight? If so, what would be the reactions of all the people who would undoubtedly be in the area at that hour? How many people would roam city streets alone at 3am? Would Galactus likely be found dancing at a trendy nightclub? That sort of thing.

This is a frequent cause of problems because a lot of times the character ends up acting in an erroneous manner because he/she really wouldn't be in a given situation.