I downloaded your model from sharecg.  A couple of things stand out.  You can reduce the polycount of your model by more than half by reducing the edge loops of the strap stitching alone.  You have 48 stitches each made up of three braided strands.  If you delete just half of the edge loops of each stitch you will go from around 81,000 faces to around 39,000 faces.  The largest, most visible areas of your model should have the mose detail and the smallest least visible parts should have the lowest detail.  With this in mind the sole parts of your flip flop could have a subdivion modifier applied and the stitching detail reduced as mentioned above would give better balance to your model.  Also look closely at your model and look for areas that will not be seen and delete those extra faces.  For example the top foot support part of your flip flop has an underside that is inside of the model that can't be seen so this can be removed, and the main sole has a topside that is covered by the foot support.  Parts of the stitching that are inside of the strap can be deleted.

When you used the array modifier to create your strap you have ended up with each of the stap "panels" overlapping indicating that you may have used an incorrect offset.  If you modeled your origin strap panel symetrically then using the relative offset set at one should give you a clean array.

Lastly before you export to .obj apply all your modifiers and use the "remove doubles" function to remove duplicate vertices.

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