I am installing a subfloor and had problems in past with seams cracking the tiles, please advise what is best way to prevent this from happening again. Thanks

What sort of subfloor material was you using... and what was the base... (IE, if this was over joists, exactly how much rigidity did they have?)

If it was over wood joists... if there is a sufficient lack of rigidity... tiles are going to crack.. they are very unforgiving. If you know it was at the seams of the subflooring, IE was this cement board? Was it glued and screwed? Was the joints taped with fiberglass mesh and thinset? BEFORE setting the tile with thinset... or were you using Mastic.

Some more information as to what you used and how you did it before so we can see what mistakes were made.

We need to know what's under the sub floor to be accurate. From what you say I am assuming the tiles are ceramic. My regular sub floor choice is Hardiebacker board which comes is several thicknesses. You glue and screw this board. There is a special screw made for backer board and I lay down a bed of thinset first and then screw every 6" in all directions, stagger your seams as you lay the backer