Delta S is the measure of entropy change within your system. Entropy is just a thermodynamic word for chaos. So if you think about things scattered all about, which side of the reaction has more "chaos". The easy answer is, whichever one has more particles. If you reaction goes from 3 separate elements like...
2 Na(s) + Cl2 (g) --> NaCl(s)
to only 1. Definitely a drop in the amount of chaos. In that case, delta S can be said to be decreasing (<0).
Also, gasses have more entropy than liquids, and liquids have more than solids; so phase changes might be a factor if there are equal numbers of particles on either side of the reaction.
Hope this leads you in the right direction!