You can try a simple random wire dangled out a window or off the balcony. But if your area is like mine - suburban, apartment complexes and private residences and some businesses nearby - you'll eventually tire of the harsh buzzing RFI. It's practically unavoidable due to street and parking lot lights, businesses with large lighted signs, neighbors with plasma TVs, and being surrounded by hundreds of poorly shielded electronic devices that contribute to a heavy fog of white noise.
So far the best compromise I've found for suburban DXing (or listening to weak local signals) is a small loop.
Take a peek at the cheap, easy to make Villard homebrewed loop:http://www.hard-core-dx.com/nordicdx/antenna/special/reducin1.htmlhttp://users.erols.com/k3mt/hla/hla.htm
That design is intended to supplement, not replace, the whip on a portable. If it offers any improvement at all with your radio on medium wave and shortwave, you might then try a more sophisticated homebrewed loop such as the KR1ST:http://www.kr1st.com/magloop.htm
My own indoor homebrewed loop is similar to the Villard design, only made from coaxial cable looped around a closet door, rather than foil or copper tubing supported on a tripod or other freestanding support. I used mine for a couple of years as a passive loop with my Palstar R30C, which has high sensitivity so a pre-amp wasn't necessary. I was generally satisfied with this until local RFI became intolerable. The large loop didn't have a sharp enough null.
Then I reconfigured it by adding a small pickup loop, also made from ordinary TV coaxial cable. This helps me null out the worst nearby offenders - flickering parking lot and streetlights. However the smaller pickup loop necessitated an amplifier. I already had a cheap amplified antenna that was popular during the 1990s. It mostly amplified indoor RFI so I rarely used it. However, I bypassed the high impedance input for the built in whip, and used only the low impedance feed with shielded cable. Even though it's not a very good quality amp, combined with the reconfigured loop it was a significant improvement over the passive loop.
Keep in mind that you'll probably get better results with a random wire slung outdoors on certain frequencies and at certain times of day or night. For example, my local RFI has very little effect above 10 MHz. So I get better reception with random wire strung along my ceiling for listening to broadcasts and hams above 10 MHz. And local RFI is usually much less intrusive late at night after neighbors are asleep, and early in the morning. So a bit of random wire is occasionally better than the loop when local RFI isn't a factor.