Traveling abroad... Advice

I'm wondering if any of you ladies have traveled abroad (or at least  been on flights lasting longer than 8 hrs?) I'll be traveling to visit my twin in Austria in mid April. I'll be 4 months by then! Already got the green light from my doctor and she suggested taking EmergenC for my immune system. I've flown there before but never while pregnant so I can't really just have a cocktail and pop a couple of Tylenol pm's... So... Any advice? Will it really be necessary to walk the aisles? What is the whole fear of blood clots about?? (This is my third pregnancy but keep in mind, my kids are 10 & 12!) so it's been a while. Any feedback is appreciated!