learn the offset filter, whenever u make something thats not exactly seamless u can adjust it with this filter.
its usually at the bottom of the filter list.
The usage is simple, if your document is 512x512 your offsets are exactly half 256x256
using different techniques to remove any lines in the center and inside edges is the last step before exporting it as a seamless image.
the KPT series or Kais Power Tools (this man rocks!) are great. My favorite program by KAi is metasynth a picture editing program designed to play the picture as a sound.!!! omg its so trippy and old but i love it.
There are 750 free plugins/filters across the net for photshop, there is aplugin manager also.
Its best not to dump a whole shebang in at once.
You will have to disect the similar filters with different names also until u find the ones u think are the most usefull.
if u have a scanner consider using it to scan stuff as a base texture.
alienskin technologys was another cool suite of filters in the 90s but i havent used them in ages.