You can use our labelling system to mark conversations as "follow up" so you'll know to come back to them later. You can use labelling in the inbox in two ways.
In a Comment/Conversation
When viewing the inbox item you'd like to label, simply click on the Add label button to apply a label. Type "followup" label - in case this label doesn't exist yet, you'll be able to create it right away. In case it already exists, you'll find it in the list.