1. Remove spam traffic in GA
You can exclude each spam source by hand but if you want to try an automatic way to do this there are tools that can help.
The tool with the biggest list of spam websites I have found so far is GA Referrer Spam blocker (free).
How you can use a GA Spam filters tool:
Click the main call to action on the page and select your Google Analytics Account where you want to apply the filters.
The tools will also ask for permissions to add these filters before starting the process.
2. Get unsampled data
Suggested tool: Google Analytics for Spreadsheets (free tool)
To unsample the data you will need to select Sampling level : Higher_Precision
See this tutorial for more details to remove sampling
Alternative tool: Supermetrics for Google Drive (paid pro feature)
Alternative tool 2: Analytics Canvas (free trial)
Analytics Canvas can reduce or eliminate sampling by using query partitioning.
3. Table Booster
The Table Booster for Google Analytics is a small plugin that can help a GA user to analyse the performance of a dimension (for example a source) on one or multiple metrics.
You can work with different tests and charts:
Here is an example where we the best traffic source by conversion rate is marked with bright green: bing / organic which has a conv. rate of 1.23% much higher vs. the avg. 0.84%.
4. Check Google algorithm updates impact on traffic
Suggested tool: Panguin (free)
With this tool you can find out if the latest Google algorithm update had any effects on your website.