Fellow DiSsers share your tips here:

Many airlines allow a second bag of shopping, I purchase a "bag for life" from a supermarket and fill it with my clothes, it will fit snugly under the seat in front.

I regularly travel to Poland and to get a better exchange rate I have set-up a bank account over there and transfer money using Transferwise. They use the real mid-market rate and charge a tiny, transparent fee for handling the payment, giving a much lower overall cost.
They are currently offering one free bank transfer:
https://goo.gl/67wY2I