If you go the herbal route, try to find a formula that does NOT contain Yohimbe. Most other ingredients they'd use should be rather safe, but check each one individually. Some are synthetic and others aren't to be taken for long periods and require on and off periods of usage.
Otherwise, a healthy diet rich in raw fruits and vegetables and exercise does well for all matters of health. Yes, including erectile problems. Some safe and extremely beneficial supplements you could take would be a multi-vit/mineral, Essential Fatty Acids (EFAs) especially Omega 3, and Lecithin. Protein is also important as it contains amino acids needed for many different tasks including getting erections. However, don't over do it because you're probably already getting enough.