• Store a reference to a user in your model.

models.py:

from django.db import models
from django.contrib.auth.models import User

class MyModel(models.Model):
    user = models.ForeignKey(User)
    ... (your fields) ...

• Force the current user to be stored in that field (when using admin)
• Force any list of these objects to be (additionally) filtered by the current user (when using admin)
• Prevent other users from editing (even though they can't see the object in the list they could access its change_form directly)

admin.py:

from django.contrib import admin
from models import MyModel

class FilterUserAdmin(admin.ModelAdmin): 
    def save_model(self, request, obj, form, change):
        obj.user = request.user
        obj.save()

    def get_queryset(self, request): 
        # For Django < 1.6, override queryset instead of get_queryset
        qs = super(FilterUserAdmin, self).get_queryset(request) 
        return qs.filter(created_by=request.user)

    def has_change_permission(self, request, obj=None):
        if not obj:
            # the changelist itself
            return True
        return obj.user === request.user

class MyModelAdmin(FilterUserAdmin):
    pass   # (replace this with anything else you need)
admin.site.register(MyModel, MyModelAdmin)

If you have MyOtherModel with a foreign key "user" just subclass MyOtherModelAdmin from FilterUserAdmin in the same manner.

If you want certain superusers to be able to see anything, adjust queryset() and has_change_permission() accordingly with your own requirements (e.g. don't filter/forbid editing if request.user.username=='me'). In that case you should also adjust save_model() so that your editing doesn't set the user and thus "take away" the object from the previous user (e.g. only set user if self.user is None (a new instance)).