The key to any trespass case is whether the accused KNOWINGLY remained UNLAWFULLY on the property of another. So, the act must not only be unlawful but also knowing. A high burden of proof. A "no trespassing" sign would take care of the "knowing" part but the act still has to be unlawful, meaning you had no lawful right or reason to be on the property. If a store owner or manager has previously told you to never come back, both the "knowing" and "unlawful" aspects have been met.
As always, I recommend that you contact a local, experienced criminal defense lawyer to discuss your particular situation.
There is a concept in the law of "curtilage", which is an area immediately surrounding a home where the public may have access. If you wanted to walk up to someone's door and knock, you wouldn't be trespassing unless you were told you couldn't be there. Front porches ate usually considered curtilage. You should be okay unless they can show you were not supposed to be there for some other reason.
This is from Wikipedia:
The curtilage is a legal term which delineates the land immediately surrounding a house or dwelling, including any closely associated buildings and structures, but excluding any associated "open fields beyond". It delineates the boundary within which a home owner can have a reasonable expectation of privacy and where "intimate home activities" take place. It is an important legal concept in certain jurisdictions for the understanding of search and seizure, conveyancing of real property, burglary, trespass, and land use planning.
In urban properties, the location of the curtilage may be evident from the position of fences, wall and similar; within larger properties it may be a matter of some legal debate as to where the private area ends and the 'open fields' start.