A very effective possibility is the announcement on the WCA-mailing-list.
If you already are a member of this list, just send a mail to:
If you want to be added to the mailing list just send an empty mail to following address:
You get a security reply, if you just answer this mail you are added to the list.
Beside this there exists also a Googlegroups Reflector with announcements:
If you start your castle-activity please spot the call via the DX-cluster and put in the info-field the WCA-number, for example WCA DL-02335.
All entries from the clusters will be checked for the signs WCA. All records containing this entry will be automatically forwarded to the Slowenian Custle Cluster S50CLS.