It would help to know what you are using now, how much room do you have to work with and budget.
A simple dipole is easy to make, cost little but it is large:
Total Antenna Length ("L"): 46 Feet, 4.6 Inches
Element Length ("E"): 23 Feet, 2.3 Inches
What is your noise level like?
A magnetic loop may be worth looking into as well but cost a lot more and or is more involved to make.