Caching can reduce expensive data transfers and improve the performance of mobile computing. In order to reuse caches after short disconnections, invalidation reports are broadcast to clients to identify outdated items. Detailed reports may not be desirable because they can consume too much bandwidth. On the other hand, false invalidations may set in if the accurate timing of updates is not provided. In (his research, we aim to reduce the false invalidation rates of cached items. Based on our analysis, false invalidation rates are closely related to clients' reconnection patterns (i.e., the distribution of the time spans between disconnections and reconnections). We show that in theory for any given reconnection pattern, a report with a minimal false invalidation rate can be derived. For practical uses, we propose to capture the reconnection pattern by sampling and develop a method to compose a near-optimal invalidation report. This method is simple and fast. Experimental results have confirmed that our method is indeed more effective in reducing the false invalidation rate than others.