Nothing special to keep the engine clean. Four factors worth mentioning:
1 - Digital pictures tend to glamorize
2 - The car has spent all its life in the benign climate of California
3 - I hand wash it twice a month and always give the engine a swipe while I'm at it
4 - I removed the worn clearcoat on the air cleaner (NevrDull + elbow grease) and recoated it with clear engine enamel and a new label at 90,000 miles.
All of these conspire to make it look better than it really is.
If anyone wants a ZIP file of the annotated pictrures to permit better quality prints to be made (assuming you contemplate doing this job) please email me and I will send the file by return.