If the magnets are just in attraction through the acrylic, then they will always allow some sliding motion. Most of the attractive force between magnets is oriented along the gap between them. Forces in the transverse directions (sometimes called restoring forces) are much lower. Usually the main thing preventing transverse (sliding) motion is friction. It is far easier to slide a magnet off a stack of magnets than it is to pull it straight away.
Think of refrigerator magnets: If a refrigerator magnet is holding up a single piece of paper, the frictional forces between the paper and the surface of the refrigerator are high enough to overcome gravity. With multiple sheets, the attractive force decreases and gravity overcomes frictional forces.
There are some magnet arrangements (such as Halbachs, as IRstuff suggested) that'll help reduce sliding, but then economics comes into play. Bigger magnets are probably the cheapest/simplest method.