I once heard a rumour that the strand that "most" genes get transcribed from gets designated as the forward strand. However one of the previous answers when this has come up on Biostars before suggests that it's about 50%-50% so that most isn't going to work too well.

See also Forward And Reverse Strand Conventions and Conventions For Designating Forward And Reverse Strands for example.

Googling this question leads to various interesting reading including this short article, which notes:

In all human reference chromosomes, as for other eukaryotes, the plus (+) strand is defined as the strand with its 5' end at the tip of the short arm (Genome Reference Consortium, personal communication, March 27, 2012).

…which seems fairly definitive and supports the centromere location discussion in the latter previous thread. But as noted in that thread, historical accidents occur and this doesn't necessarily hold for various particular species…