Question
we are one of 4 houses set back off the road in a rural setting and want ideas how to design the entrance at the road. one side must include a high retaining wall as the driveway slopes upwards. any ideas

Answer
I used to live on a similar community in Virginia...there were four of us on a cul-de-sac, in a rural part of Albemarle County.  The neighbors across the street ran a 600+ acre cattle farm.

I'd always thought of how we might set our little road out as being it's own enclave.  Then, after moving back west, one of my first clients was doing a series of cluster large-lot subdivisions on successive cul-de-sacs, and asked me to create several rural-themed entries for his horse ranchettes.  We had to design about 8 entries, each one unique.

All this to say, I think I understand what you are hoping to achieve.  My original desire was to point you to a website full of images.  But alas, I couldn抰 point my browser on just one.  I抣l see if I can type it out, instead -

It will be difficult to give specific suggestions, as it sounds like you'll be grading the road and adding retaining walls... If at all possible, try and tie everything together (fence/entry/retaining walls).  It presents a more finished & purposeful effort, not just "slapped together"

I would suggest several other things in the design process ?br>
1.)   Continuity is the key to setting a sense of community.  Any sort of monument or structure gives a sense of 揳rrival?to the neighborhood, and a perception of 揺ntry??kind of like a front door to a house ?big & bold is better than timid and demure.
2.)   Be careful of tired & over-used elements like wagon wheels, plowshares, or old tire rims.  But, creative & adaptive re-use of whatever is at hand IS a plus.  I抳e seen some entries made up of old milk cans, gnarled lumber, or bails of rusty barbwire.  I will assume you desire 揷lassy?over 搆itsch?
3.)   If it the area or farm has name, use it.  The goal is not only community 揺ntry? but also identity.  Avoid using current resident抯 names (security & ego issues)匨aybe something historical or geographical.
4.)   It may be worth it to add lights to the entry structure, if possible.  Rural locations are notoriously difficult to find!  Just ask a friend who has tried to locate your cul-de-sac at night, or the UPS driver seen zooming up and down a rural lane.
5.)   Match any post or entry arch material to the retaining wall materials (Again, continuity).  So, if you are doing brick, ledge stone, or stacked stone retaining walls, have the bases of the entry pilaster(s) or mailbox shelf of the same material.
6.)   At each estate/lot entrance, again match the materials of the entry monument or pilasters.  Just at a smaller scale.  
7.)   Same thing with any tree-lined roadway ?use all the same species of tree & large shrubs, if possible.
8.)   If the mail is delivered to a central row of mailboxes at the project entry, try and incorporate them into the entry structure/arch/pilasters, or again, use the same materials.
9.)   Uniform fencing style throughout ?all 2/3 split-rail (cedar/vinyl, etc.)  Match colors among the lot owners, if possible.
10.)   Tie the fencing or retaining wall in to the entry structure, if possible.  This is the power (and sometimes the 揷urse? of a covenant that is part of the subdivision process卆n attempt to create a sense of neighborhood and continuity of design. It pays off in land values and a perception of community exclusivity.  I get paid alot of money to help developers figure this out!  
11.)   Not everybody will desire this method, as this runs contrary to may folk抯 belief of 揥hat抯 mine is mine; let me alone to do whatever the heck I want? but SOME cooperation is needed to create an ambiance of community.
12.)   If you are looking for specific materials suggestions, see what others in your area have done.  Take pictures as I have found, "The weakest (printer) ink is stronger than the best memory攨
13.)   Last, try typing in key words at images.google.com phrases like 揈ntry Structure? 揊arm Road entry? 揻arm road gate? etc., and see what pops up卪aybe you抣l have better luck than me.  

(Remember, this is somebody ELSES work, and not free for the taking; most folks are happy to give permission to copy it, however).

Finaly, a nice little example:
http://www.nice-touch.com/LANDSCAPING/LandscapeFarmEntry.jpg

Sorry I don't have any of my own on the web, yet...and I didn't even begin to mention COST!!

Hope it helps.  Marc