Add a custom property to the site node

Here is an easy way to add and retrieve a custom property to you site in the .config file.

Here is an example with my custom property SiteUID

1. Add this property to your site tag:
   

   <site patch:after="*[contains(@name, 'login')]"
       hostName="uk.xxx"
       targetHostName="uk.xxx"
       name="UK"
       siteUID="MyUniqueID"
       virtualFolder="/"
       physicalFolder="/"
       rootPath="/sitecore/content/United Kingdom/Website"
       startItem="/Homepage"
       contentStartItem="/sitecore/content/United Kingdom/Website/Homepage"
       database="web"
       domain="extranet"
       allowDebug="true"
       cacheHtml="true"
       htmlCacheSize="10MB"
       registryCacheSize="0"
       viewStateCacheSize="0"
       xslCacheSize="5MB"
       filteredItemsCacheSize="2MB"
       enablePreview="true"
       enableWebEdit="true"
       enableDebugger="true"
       disableClientData="false"
       language="en"/>

2. Create an extension method:
   

   /// <summary>
   /// Retrun the site unique ID
   /// </summary>
   /// <returns></returns>
   public static string SiteUID(this SiteContext site)
   {
       Assert.IsNotNull(site, "Site cannot be null");
               
       try 
       {         
           string uid = site.Properties["siteUID"];
           if (!String.IsNullOrEmpty(uid))
               return uid;
           else
               return site.Name;
       }
       catch (Exception)
       {
           return site.Name;
       }
   }

3. Use this property in your code: (don't forget to add you using to the class with your extention method)
   

   Sitecore.Context.Site.SiteUID()

Write you messages in a separated file

It is not always easy with a sitecore "out of the box" to know witch messages come from your custom code and witch messages come from sitecore himself.
Here is a small tip to write you custom messages in a separated file:

First of all, we will prefix all our messages with a custom prefix "SH - " in my case:

Log.Info("SH - My message", typeof(MyObject));
Log.Error("SH - My message", typeof(MyObject));
...

And then to write it in a custom log file change in the web.config in the log4net section:

<appender name="MyFileAppender" type="log4net.Appender.SitecoreLogFileAppender, Sitecore.Logging">
    <file value="$(dataFolder)/logs/mylog.{date}.txt" />
 <filter type="log4net.Filter.StringMatchFilter">
    <regexToMatch value="^SH .*" />
    </filter>
    <filter type="log4net.Filter.DenyAllFilter" />
    <appendToFile value="true" />
    <layout type="log4net.Layout.PatternLayout">
    <conversionPattern value="%4t %d{ABSOLUTE} %-5p %m%n" />
    </layout>
</appender>

You will need to adapt the regex with your custom prefix of course.

And second point after the logFileAppender of sitecore add you custom one:

<root>
    <priority value="DEBUG"/>
    <appender-ref ref="LogFileAppender"/>
    <appender-ref ref="MyFileAppender" />
</root>

You have a lot of example of what you can do with log4net here: http://logging.apache.org/log4net/release/config-examples.html

EDIT: You have an alternative syntaxt and more info about the filters here https://sitecoreblog.blogspot.be/2017/03/log4net-add-stacktrace-in-logs.html