What do you mean by smoothly? The majority of CPA networks are hot on rebill offers. This is where the big money is made. Dieting, bizopp, teeth whitening, colon cleansing, skin treatment... If you can break in to those markets, you're breaking in to riches.
Pretty much any niche a reputable CPA network has listings of advertisers in is a good niche. The niche you go into may also depend on your budget, and how much you're prepared to lose upfront before you see profit, if your doing PPC advertising for example. It may also depend on you and which niche you think you could work well in. Ultimately its up to you to try them and test them and see what works for you.