Chipmunks, bunnies, woodchucks, and deer are my biggest thieves and while I still have troubles (the endive babies I planted out two days ago are already chewed to nothing) there are a few solutions that have, at least, slowed the problem.
A fence, but not just any fence, is the first step. If you are going to stop critters, it needs to be small enough to keep the chipmunks out but big enough for the big guys. I have found that the relatively invisible deer fences are good for not only the deer, but the holes are small enough to stop the little guys. My best tip is to fold the bottom 12 - 18 inches of the fence out away from the garden, lay it flat on the ground and bury it under some mulch. This is much easier than trenching the fence in to the same depth to stop the digging critters from going right underneath. Nothing is more satisfying that watching a munchy woodchuck walk up to a fence and try to dig underneath it unsuccessfully and know that if he just backed up up 18 inches he would be successful – but, he never does.
City dwellers have to deal with two legged thieves who prey on summer confections. The prettier your garden the more likely you are to take a hit. For many years I planted and maintained a big urban property and there were things I learned along the way. I hope these few suggestions will to make your plantings less of a target.
Try to make your pots heavy – or even just look heavy. It doesn't have to be immovable to be un-attractive, it just has to give that impression. Big pots often make a better statement in front of a home anyway. But planter size won't deter all larceny – a pretty plant will still be an individual target. So, consider the planting and aim for things that aren't too special or unique. Don't bother with things like canna lilies – there is only one big bloom and if some one comes and cuts it off to add to their own arrangement, there isn't a back up plan. You are far better to opt for plants with lots of blooms so when a few go missing it won't wreck the whole composition. Even better is to try to plan a planting that is more about the sum of all the parts rather than making one or two really 'wow' plants carry the day. The 'wow' plants are always the first to go missing. Tall grasses and really great coleus and textural plants have tended to work for me (with littler plants added in).
Skip the garden ornaments entirely. Fern, over at Life on the Balcony, recently posted about her missing plants and ornaments (the amusing commiserative comments are worth a read) and she reports that, ironically, thieves even stole her American flag.
In another interesting missing plant post, suggested solutions veer towards the extreme. A Daphne shrub was strapped to some deeply buried heavy pottery and barbed wire and a magnolia tree was chained to a neighboring fence. Personally, I am not sure I am ready to go that far but I understand how frustrating the whole thing can be. Have you had plants stolen? How did you remedy the problem?