The redline made easily be one of the most important concepts for a poker player who plays well, yet has difficulties finishing with a profit at the end of the day. Locating the leak is almost always more challenging than plugging it up. Taking a look at your redline may well help you discover a leak in an area of your play where you probably never suspected that there could be one.
Let's start at the beginning though: what exactly is your redline? In poker analysis software like Holdem Manager and Poker Tracker, your showdown winnings are presented in the shape of a graph. On this graph, there is redline which represents your non-showdown winnings. If this redline exhibits an upward tendency, then your non-showdown winnings are okay in the sense that you do win more non-showdown pots than you lose. For many people though, this redline slopes straight downward. Lately, there has been plenty of talk on various poker forums about the importance of the redline. Apparently, the fact alone that you redline slopes southward doesn't mean that you are indeed hurting because of it. If it does show a sharp downward fall though, you've probably found the culprit you were looking for.

In such cases, instead of giving up your old poker rakeback or pokerprop deal and looking for new ways to diminish your poker rake and to secure extra value, take a look at the following factors (which are probably at fault for your downright abysmal redline. To be overly simplistic: a downward sloping redline is caused by the fact that you put money into the pot and then you fold, and you do this often.
Abusing the continuation bet can be one of the causes leading to a less than ideal redline. Playing out of position is definitely a mistake that will result in the folding of many pots you have money in.
Playing your draws passively and taking way too long to realize that you’re beat - this one is a classic beginners’ mistake. Folding way too many flops - I understand that you may be going through a bad run starting hands-wise, giving up way too many flops for way too long is definitely going to have an effect on your redline though. Check-calling is never really a good idea, but when you're doing it on week made hands like top pairs, and hands which are essentially two pair draws, you're basically asking for it.
Turning passive after an aggressive first stint in a three-bet pot is like handing your opponents a written request to jump all over you and to trounce the living c*** out of you.

If you recognize any of these mistakes in your own play, consider the possibility that your ailing red line is caused by these mistakes, and try to do away with them. Reanalyze your profits and your redline after you filter out the mistakes.
If you are a winning poker player and you realize you have a downward sloping redline, don't mess with it. As they say: there's no use in tinkering to fix something that's not broken, chances are you'll only end up making less money.