have a look at the TV table I built a couple of months back and see if that is the sort of frame you are looking for. The main section is 32mm laminated chipboard which gets around the expansion/contraction issues that Charles mentioned. That by the way is a seasonal thing so you will see it expand across the grain in summer and contract in winter. As the wood dries over time, it contracts more than it expands. I have a 100 year old house where the doors are made like this, and some of the panels no longer meet the frame regardless of the season.
Anyway, if the TV table has the same type of frame you are after, then it is more a matter of being careful and accurate than anything else. I have a digital protractor and use that to set the angle on my saw bench. If that is not handy, I also have a 45 degree builders square that I use. 1/10 of a degree variation will show up in the mitre. I make sure that I am measuring the saw blade only and avoiding touching the teeth as they protrude out from the blade.
The other trick I use is to clamp a scrap of wood on the cross-cut fence and run it through the saw to make a temporary fence with zero clearance. That way I know exactly where the blade is cutting and can line up the marks on the real piece exactly. This is a bit difficult to explain without a picture, but I'll try anyway. I put the real piece upagainst the fence but clear of the blade. Rest the scrap on top of the piece and against the fence, making sure it is extended far enough to be cut by the blade. Clamp it there for a tight sliding fit with the real piece and then cut through. I don't cut it off completely - juse raise the blade enough enough to make a channel cut through. Sometimes you use the inside edge and sometimes the outside edge of the blade. Now that I know where the blade is cutting, I line up my pencil marks with the appropriate edge in the temporary fence and cut straight through. I will typically err on the side of caution and make it a fraction longer so that I can gradually take it down to the required size, but I have found this to be fairly accurate.
I guess this is similar to making up a sled which is configured for 45 degree mitres, but I haven't got around to doing that sort of thing, and by using a temporary fence I know that I have a cut which is exactly the way the saw is cutting on that day. The kerf exactly matches the blade now, not as it was last year, or for another blade, etc. etc.
Something else I have done is to round over the sharp edge of the mitre slightly with a file. This not only makes it a bit more user friendly without the sharp edge, it can also hide minor misalignments with the mitre.
Hope this helps,