mRNA transcripts can be localised by several methods:
Negative regulation, where mRNA is selectively degraded in regions of the cell that are not to express that protein, can be done by increasing the erosion of the poly(A) tail, leading to the convention decay pathways being activated.
Random diffusion and anchoring interactions occur where the mRNA becomes trapped on a localised protein, allowing the protein to then be expressed. This is reliant on diffusion, so the mRNA transports throughout the entire cell until it comes into contact with the localised binding protein.
Active transport can be by the mRNA binding to a protein that attaches to a motor protein and actin filament. This is employed in yeast to localise ASH1 transcripts to the daughter cell, controlling the mating types.
