I book about 40 nights in Europe a year and have for the last few year have had as Baldrick says, "a cunning plan".
I have a specific email address for hotel bookings with its own calendar.
I have joined a number of the above mentioned sites for email subscriptions +signed up free membership with a number of chains over there (Accor/Ibis, Intercity, Travelodge, Leonardo, etc).
I fill in my locations on the calendar and usually list hotels I have stayed with (and liked!!) at the city + any others I find that are well rated in the location.
I have preference for hotels within walking distance of the main city station. No taxi needed, can often leave bag at hotel for quick pickup after checkout and if not, €5 48 hour lockers at the stations.
My minimum standard is air con (must in summer due to humidity); inclusive wifi and breakfast and at least double bed as I am tall (double rooms usually not much more than a single and my lady sometimes joins me at short notice anyway).
I usually travel on a Eurail pass which are excellent value and no res required/ walk up - except SNCF's disgraceful treatment of passes on TGV's (a totally different issue that I could rant about for pages and so just avoid France to keep my blood pressure down).
i usually have my dates set 6+ months in advance so can then wait for deals and fill in the confirmed booking as I go.
Example: Recent August trip included Austria section based at Linz and Vienna. 6 nights in school holidays, so likely to be expensive. Pre-booked some rooms on booking.com at about €70 p.n. With free cancellation option.
The Accor/Ibis special mentioned by someone earlier in the thread popped up on the emails, so I swapped the nights for the IBIS hotels outside the station and got them for €38 a night
Estimated average room rate for average 4 star hotels this year is €46 for 38 nights in Europe and £32 in UK (though I cheat in London as I stay with family). Total taxis taken in Europe this year - none. About €25 spent on station lockers for luggage usually because I was too lazy to leave the bag at the hotel or was visiting sonehwere for half day before proceeding to another city.
Also paid €45 for a couchette supplement on a CNL sleeper train 3 times with my Eurail pass.
Total savings well over $3,000 which is well worth the time used to plan and book.
Cheers
Chris