Before going offline you have to make sure that everything is in your local repo, which is required while working offline. Running "mvn dependency:go-offline" for the project(s)/pom(s), you intend to work on, will reduce the efforts to achieve this.
But it´s usually not the whole story, because dependency:go-offline will only download the "bare build" plugins (go-offline / resolve-plugins does not resolve all plugin dependencies). So you have to find a way to download deploy / test / site plugins (and maybe others) and their dependencies into your repo.
Furthermore dependency:go-offline does not download the pom´s artifact itself, so you have to dependency:copy it if required.
Sometimes - as MaDa wrote - you do not know, what you will need, while being offline, which makes it pretty impossible to have a "sufficient" repo.
Anyway having a properly filled repo you only have to add "<offline>true</offline>" to Maven´s settings.xml to go offline.
Do not change the Maven profile (id) you used to fill your repo, while being offline. Maven recognizes the downloaded artifacts in its metadata with an "identity", which is bound to the profile id.