I cannot say anything about the Ricoh lens, never tried one.
Manual focusing does get easier. But it is important to remember that the k-x, like most (if not all) modern dslrs was built with auto-focus lenses in mind. The view finder in particular is optimized for slower, dimmer auto-focus lenses.
There are a few things that you can do that will help.
1) Practice! Practice! Practice! It really does help. And it will not be an "aha" moment. You will just find that over time you get better. At least I did. I had to teach myself that "close" was simply not good enough.
2) You already have the magnifying viewfinder, I assume the OME53. Best purchase I ever made, would not go out without it.
3) You can try a replacement focusing screen. They are available for the k-x on ebay or from
katzeye. Search for pentax focusing screen and you will get lots of hits. I have read several long technical explanations and I still do not think I'm qualified to explain why but a focusing screen made for a manual focus camera will work better with manual focus lenses.
4) Not all lenses are created equal, manual focus or auto-focus. Try the lens database
here for some information on lenses. Some are simply better than others. Some just do not take sharp pictures.
5) You need to have a good understanding of depth of field. Are you using the Ricoh wide open or stopped down? Trying to manually focus a wide open f/1.7 lens on a tree will be frustrating. The depth of field at f/1.7 would be only inches or less. So some part of your photo will be in focus, just not all of it. That may be why your sign pictures are working since the area you are trying to focus on will be flat with little depth of field.
6) I know nothing about your Ricoh but be aware some lenses cannot be stopped down on modern cameras and will only work wide open. I'm thinking of M42 lenses without a manual/automatic switch. I doubt this is true for yours but you should check. In this situation you are always working with a very narrow depth of field no matter what you set the camera at.
7) You can also try focusing with live view and zooming in. This works better with a tripod for me but you can do it hand held as well. You might want to try this to test the lens and make sure it is functioning correctly. I use this method all the time for macro pictures.
8) Be aware that your camera may be front focusing or back focusing. In small amounts this is not noticeable with the kit lens or other slow lenses. But when you try to focus with a fast lens (f/1.4, f/1.7) the slight error becomes apparent. If you are getting good shots of flat things like signs this is not likely your problem but you should be aware of it.
9) The focusing mechanism of a lens makes a lot of difference for me. I have some with very long 'throws' where fine focusing is very easy and others where the focusing ring seems to make finding that right spot harder than it should, it just seems to go past instead of hitting that sweet spot.
10) It is also important to learn how to manually focus. I work my way back and forth at ever decreasing intervals until I can see the focus going from sharp to blurry. Take a look at this thread for some tips on
manually focusing.
11) You can also use catch-in-focus on most manual focus lenses. I don't use it much but many report it works very well for them.