Pilot log books have a section at the back for logging instrument time.
In fact the CASA Pilot's Log Book titles this section:
FLIGHT SIMULATOR, SYNTHETIC TRAINER AND IN-FLIGHT INSTRUMENT EXPERIENCE
and is used to show your IFR recency both in flight time and navigation aids.
Pretty well sums it up for providing the required crossed "I's" and dotted "T's".
As an aside, irregardless of what the rules state verbatim, if you want to really keep your instrument skills, if actually flying, you should do it under the hood and take a safety pilot with you if not in actual IMC and flying under the IFR.
