I've been working on a bunch of tiles lately and I just figured out how useful layers can be in photoshop. So, I give you a brief runthrough of how to make all the necessary transitions between two tiles, ie. a grass and a dirt tile. (sorry, I don't have pics, i just got excited and wanted to share it with people who might be new to photoshop or making tiles or just never thought of it, like me.) OK, this will be very basic just to get the point across. You have two tiles, A and B. You need to make four 1/2 transitions, that is, one half A and one half B, in each cardinal direction. Then you need eight 3/4 transitions, the corners, so you can make paths that turn, etc. (thats four tiles that are 3/4 A and four tiles that are 3/4 B) Here is how to make all of these tile with just 7 layers. One of the full tiles, lets say A, should be under all other layers. Make four duplicate layers of B above the one A and two more A layers above those. Make a layer mask for the top 6 layers. For the four B layers, make the four 1/2 transitions with their individual layer masks. You will have two horizontal and two vertical transitions (up, down, left, right. now those four are done). Now, pair up one horizontal masked layer B with one vertical masked layer B. That will give you one 3/4 B transition. Just pair up the rest like this (horizontal 1 with vertical 1 then vertical 2, then horizonal 2 with vertical 1 then vertical 2...) Now you only need the four 3/4 A transitions. To make those, copy either the two vertical or two horizontal layer masks you have on the B layers and apply them to the top two A layers. Unless you have perfect 1/2 proportions for your layer masks, you will need to invert the masks you just copied onto the top two A layers to get the next four transitions correctly. Once you do that, pair up the A layer masks with two B layer masks you did not copy to get then next four. This process took me about five minutes once I knew what I was doing! Good luck, hope this helped!

You could do that with 5 tiles... =)

Top Layer

TileB x 3 - Other Corners, Darken

TileB - One Corner, Normal

TileA - Normal

Bottom Layer

Hide and unhide the ones you want!
Yay, 5 Layers!, but maybe that's more work =)