On the contrary I do understand my adjacent sectors very well. And you're right, that is absolutely essential in providing a good service.
Perhaps my understanding of the mach/IAS is not quite correct then. My understanding was higher level cruise (depends on a/c type but usually above FL300) was done in mach. Maintain a constant mach during descent (e.g. 0.795), during descent on a constant mach groundspeed will increase slightly as you get lower. IAS, even though not flying with reference to it at higher level, will be fairly low at FL400/M0.795, something like 220kts or maybe less. This will also increase gradually as you descend on a constant mach. Then on reaching a specified IAS (say 290kts) you switch from flying mach to flying IAS. Descent on IAS will then cause groundspeed to decrease as you descend. You maintain that IAS (provided no other factors such as turburlence etc require you to alter it) until reaching lower level where any other procedural speed restrictions may kick in (e.g. 250kts at FL100).
That is my understanding provided there is no intervention from ATC. If it is incorrect then please do say as this is what I use on a daily basis to separated a/c and I would really appreciate further understanding. As Denti explained, my instruction was intentionally issued at high level, and in good time for the pilot to plan a descent at not above 270kts. Since at high level he is highly unlikely to be flying faster than 270kts, the speed restriction wouldn't really kick in until he is at say FL350(ish) (if flying at M0.85 that is approximately where I'd expect it to be). Bare in mind I never asked the pilot his mach number to begin with as I couldn't care less. I just needed him slower below FL300 as he descends and meets other traffic.