I use this for every package I install. Although for some you'll run into some problems or extra steps are needed.
For example:
I can't figure out how to install TeTex into a fake root, so I use steps similar to
this Slackware compile script to install it all to /usr/share/texmf and then create a package. Although I can't say I care for this way since you have to append the bin directory to PATH and some other things which you can find in the profile.d script in there. Not to mention it gets even messier when you add programs that use TeTex such as JadeTex which modifies some files and I found I had to change some things in one of the config files in the JadeTex section.
If you're installing GNOME, you'll have to run some extra commands before starting GNOME.
gconftool-2 --makefile-install-rule /etc/gconf-2.0/*.schemas
scrollkeeper-update -v -p /var/lib/scrollkeeper
If I have no clue, I either look at
Slackware source and for many packages (but not any of the GNOME ones that use install schemas or scrollkeeper files, but you can just use --disable-schemas-install to prevent it from registering the schemas, use DESTDIR to fake install it, and then run the above commands (you can also prevent it from running scrollkeeper-update with a sed command to comment that out, but it doesn't really matter)) they have a way to install them into a fake root. Or I look at a Gentoo ebuild and if I can figure out what the hell it says I can figure out how to fake install it that way too.
Other than that, I find that I can cleanly install every other package using DESTDIR (most command), install_root (the one for glibc I believe), ROOT, INSTALL_PREFIX, or some other kind. Usually I just look in the install: section of the makefiles and unless it just installs to like ${bindir} without ${DESTDIR}${bindir} or some other variable prior to the location, you can usually find the variable you need to set during make install pretty easily. Otherwise Gentoo may have a patch for using DESTDIR or Slackware scripts can tell you how to install the package manually.
It may all seem complicated in my post, but most packages use DESTDIR and very few either don't have that capability or use another variable.
