What mechanism allows a ribosome to translate or move across mRNA during translation?

In eukaryotes, there are are three different types of protein factors that help during translation.  Initiation factors, elongation factors, and termination factors.  For your question, you are concerned with elongation factors.  In eukaryotes there are I believe three factors involved.  Basically, one loads the energy providing molecule (GTP) onto another, a third allows binding of the energy molecule to the tRNA, the dephosphorylation of which allows the attachment of the amino acid to the protein strand.  The ribosome has a subunit that then expends one GTP to shift the protein strand downstream and the whole process starts over.  Sorry I didn't go into too much detail, but if you need more, just google elongation factors.