Well, knowing how your sugar levels react to travelling in general is a good start, as well as if you know the details of the travel you're doing - how much will be in transport where you're sitting, how much dashing from point to point, what sort of luggage/baggage is involved? I would always recommend having some form of sugar with you when travelling, especially if you're not confident of having places you can buy top-up snacks as you travel. My bag usually has assorted packets of sugar/biscuits (collect from hotels, cafes etc.) for dire situations, and at least one small (200ml) pure fruit juice (apple tends to be naturally sweetest, but you might have other options available - just check how much sugar they actually have). Chocolate, candy/sweets, glucose tablets - all viable options depending on personal preference and circumstance (chocolate not so good on hot days, for example).
On top of carrying sources of sugar/snacks, I'll also often reduce my insulin (long and short) and let my sugars run higher than usual, just keeping an eye on them and making minor adjustments if necessary. It's much easier if you're on a pump; I'm always wary of using pens in public, but sometimes it's not an option. Again, your mileage will vary depending on how you react when your sugars are at different levels, so you might find you have less margin for 'error'.
