I have 4 beds that are 4x8 and 2 beds that are 4x12. The bigger beds and one of the smaller ones are made of ceder that I got from Menards. I used 2x8 (they didn't have 2x10's) on them. The three original beds are made of non-pressure treated pine 2x10 that I coated with linseed oil. I put 4x4's in the corner of all the beds to attach the 2x's to. I stuck them down about 6" past the bottom of the side rails to hold things in place. I've never had a problem with them moving. In one of my 4x8's, I put a "cross-beam" in the middle, giving me two 4x4 beds. I've never had a problem with any of the other beds bowing.
My plan (if I have time) for this spring is to install drip irrigation. I have all of the parts, just no time.
I originally left 48" between all the beds so I could mow with my Gravely (42" deck") between them. Then I bought my JD and rarely mow with the Gravely any more. I think I'm going to just use wood chips (I have a PTO
chipper) in between all the beds so I don't have to mow at all between them.
My layout of the beds has all of my 4x8's laying side by side. Then I have the two 4x12's laying end to end, perpindicular to the smaller beds.
Soil is the key for these. I'm on my third year of these beds and haven't had a ton of luck gardening so far. I bought the "super gardener mix" of soil and compost from the local mulch company. I wasn't impressed with it. Hopefully you can get better soil/compost in your area.
One of my 4x8's is dedicated to strawberries. I'm thinking of taking them out of the bed (so I can use it for something else) and transplanting the strawberries into a movable strawberry planter. I think they're a shallow root plant so they should do well in a planter.
Good luck and post pics!!