How do I properly bill for travel costs?

Do you actually mark up your travel so there is Income?  I handle my Travel as a Reimbursement.

Years ago my Accountant had me set up the Invoice "Travel Expense" item to go right back into the Travel Expense Account.  So basically every time the Client pays the Invoice, that portion just lowers my company expenses the same amount I had put in.  So this portion of the Client's payment does not show up as Income.

If you're asking how do you show it on the invoice.  As I said above, I have a Travel Expense item I just add to the invoice and it's pretty easy to figure out my Expenses, but I could run a report for the dates involved if I really needed to.  If you're tracking it a Project, it should be pretty easy to get the total for individual jobs.