To re-use your proposal templates, you can either duplicate a past one or save a past one as a template.

To duplicate a proposal is very simple.  Just select 'Duplicate Proposal' from the dropdown in your proposals dashboard.

From there, you just need to select the project to add it to.

If you know that you'll want to re-use this proposal multiple times, you'll likely want to save it as a template for easier re-use.

To do so, just select 'Save as Template' from the dropdown in your proposals dashboard.  You'll be prompts to add a name for your template.

Once you've saved it, you'll be able to select it from your available templates the next time you create a proposal.