Recently, Siamese network based trackers have attracted growing popularity in visual tracking, which tackle the tracking by template matching between the initial template and successive search regions. The initial template patch is generally encoded into a convolutional feature for matching. However, the limited representational capability of the template feature limits the tracking accuracy. Besides, this fixed representation also fails to adapt to the target appearance changes. To alleviate these issues, we improve the Siamese trackers by introducing temporal correlation and channel decorrelation mechanisms. On the one hand, we consider the channel-wise correlations between the initial and historical template features to adaptively aggregate informative channel-wise representations for template update. On the other hand, we propose a decorrelation regularization to weaken the channel-wise correlations of individual template features. By end-to-end training, we learn a more complete and adaptive template for accurate object tracking. We demonstrate the generality of our approach by applying it to two prevalent Siamese trackers, i.e., SiamFC and SiamRPN. Extensive experiments on seven benchmark datasets verify the effectiveness of our method.