Make sure the timezone (Olson) data is up to date.
Olson timezone data should be regularly updated in the trunk. If the data is out of date, follow the instruction described in the readme file.
Once the timezone resource file is generated by the process above, the file should be checked into zoneinfo64.txt in the trunk.
Update script code enums with new codes from ISO 15924, send a notice for them as public API (born @stable) to the icu-design list.
Starting with ICU 55, add script code enum constants only after assignment of Unicode script property value aliases; otherwise the names of the constants may become confusing.
Soon after a Unicode release, work to assign property value aliases for scripts that are known to be added in the next version, then update ICU.
Constants for script codes like Latf and Aran that are not subject to separate encoding may be added at any time.
See details in icu/trunk/source/data/unidata/changes.txt and keep the list of script codes not yet in ICU up to date there.
For new codes see http://www.unicode.org/iso15924/codechanges.html (check for recent changes with the code highlighted in the left column and "Add" in the right column) and http://www.iana.org/assignments/language-subtag-registry (Search for "Type: script" and look for recent "Added:" dates.) Discuss what to do if these two sources are out of sync.
Update ICU data from CLDR.
Add a tag to the CLDR repository that specifies exactly the CLDR data files and tools that were used to create the ICU data for this ICU release or milestone. Document the tag name in the ICU Readme file. Verify that ICU data can be successfully regenerated using the GA ICU code and the tagged CLDR.
If CLDR already has an appropriate tag, just use it.
An example of a tag in CLDR: http://unicode.org/cldr/trac/browser/tags/icu432milestone
Update MeasureUnit Code
Make sure the ISO 4217 numeric code mapping data is up to date.
See the instruction in http://source.icu-project.org/repos/icu/tools/trunk/currency/readme.txt
ICU Processes and Procedures > Throwing the Big Red Switch: How to ship ICU > Release & Milestone Tasks >